Salt-Induced Thermoresponsivity of Cross-Linked

Subscriber access provided by READING UNIV

Article

Salt-Induced Thermoresponsivity of Cross-Linked Polymethoxyethylaminophosphazene Hydrogels: Energetics of the Volume Phase Transition Valerij Y. Grinberg, Tatiana V. Burova, Natalia V Grinberg, Vladimir S Papkov, Alexander S Dubovik, and Alexei R. Khokhlov J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b11288 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

1

Salt-Induced Thermoresponsivity of Cross-Linked Polymethoxyethylamino-

2

phosphazene Hydrogels: Energetics of the Volume Phase Transition

3 4

Valerij Y. Grinberg*†, Tatiana V. Burova‡, Natalia V. Grinberg‡, Vladimir S. Papkov‡,

5

Alexander S. Dubovik†, and Alexei R. Khokhlov§

6 7

†

8

4, 119334 Moscow, Russian Federation

9

‡

N.M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin St.

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,

10

Vavilov St. 28, 119991 Moscow, Russian Federation

11

§

12

Moscow, Russian Federation

M.V. Lomonosov Moscow State University, Physics Department, Vorobyevy Gory, 119992

13 14 15 16 17 18

*Corresponding author: Valerij Y. Grinberg, Tel. +7-499-135-07-28; fax +7-499-135-50-85;

19

e-mail [email protected]

20 21

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 39

1

ABSTRACT: Biodegradable hydrogels of cross-linked polymethoxyethylaminophos-

2

phazenes (PMOEAP) of various cross-linking density and apparent subchain hydrophobicity

3

were investigated by high-sensitivity differential scanning calorimetry and equilibrium

4

swelling measurements. The volume phase transition of the hydrogels was found to be

5

induced by salts of weak polybasic acids. The transition parameters were determined

6

depending on pH, the phosphate concentration, cross-linking density and apparent

7

hydrophobicity of the gels. The transition enthalpy increased three times and reached 60 J g-1

8

at the phosphate concentrations 5-100 mM. The transition temperature decreased by 60 °С

9

when pH changed from 6 to 8. A decrease in the transition temperature (by ∼20 °C) was

10

achieved due to incorporation of 9.4 mol% of some alkyl groups into the gel subchains. The

11

classic theory of the collapse of polymer gels coupled with the data of protein science on

12

hydration energetics for various molecular surfaces reproduces correctly thermodynamics of

13

the collapse of PMOEAP hydrogels.

14

2 ACS Paragon Plus Environment

Page 3 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

INTRODUCTION

2

“Smart” or “stimuli-sensitive” polymer hydrogels are capable of reversible volume phase

3

transitions in response to minor variations in parameters of physiological medium. In

4

particular, the transitions can be triggered by changes in temperature1-4. Hydrogels of poly(N-

5

isopropylacrylamide) swell at room temperature and collapse at temperatures above 33 °С

6

that corresponds to the physiological temperature range1, 5-8. Introduction of comonomers

7

capable of formation of specific reversible bonds with target ligands (drugs) can provide an

8

effective control over the ligand binding and release by the gel2, 7, 9-15. Such systems

9

demonstrate promising results enabling design of devices for controlled drug delivery16-19.

10

Biomedical application of the known to date thermoresponsive synthetic polymer

11

hydrogels, including gels of N-isopropylacrylamide copolymers, meets serious limitations

12

because of their apparent toxicity – such synthetic polymers are in general not biodegradable.

13

The organism’s response to such polymeric components, and their excretion from the body

14

present a challenge. For this reason new synthetic biodegradable stimuli-responsive polymers

15

are of vital importance. It is known that polyaminophosphazenes are naturally biocompatible

16

and biodegradable polymers, namely they can be hydrolyzed in a controlled manner in

17

physiological aqueous medium20-21. Some of polyaminophosphazenes demonstrate

18

thermoresponsive behavior. Mostly, these are polyaminophosphazenes obtained by

19

exhaustive aminolysis of polydichlorophosphazene with a mixture of α-amino-ω-

20

methoxypolyethylene glycol and amino acid ethers8, 22 or alkylamines23-24. Aqueous solutions

21

of these polyphosphazenes are phase separated upon heating. The phase separation

22

temperature is approximately constant within a rather wide range of the polymer

23

concentrations24-25. However, it can change from 25 to 100°С depending on the

24

polyphosphazene chemical composition23, 25. Moreover, the transition temperature increases

25

significantly in acid medium22. The gelling ability of polyaminophosphazenes upon heating



1

to the physiologic temperatures as well as their biodegradability20-21 define a promising

2

potential of using these polymers in drug delivery systems26-28.

3

Page 4 of 39

At a high enough polyphosphazene concentration (~10%) the phase separation leads to

4

gelation21, 29. The gels produced by incomplete phase separation of aqueous solutions of

5

polyphosphazenes are thermoreversible30-33. They are stabilized by physical bonds and,

6

accordingly, demonstrate rather low mechanical properties26. These gels are intended for

7

injected drug delivery systems since they are readily formed at physiological body

8

temperature but melt at room temperature. The polymer concentration in such systems

9

reaches 5−10%, which is undesirable from the safety standpoint. Besides, the

10

thermoreversible gels are not suitable for transportation of drugs through the gastrointestinal

11

tract. These hindrances could be overcome by using chemically cross-linked gels with

12

thermoresponsive properties. Such gels seem to have a concentration of approximately 1% at

13

room temperature and more than 10% at body temperature. At the same time, their

14

mechanical characteristics can be easily tuned to a necessary level. Therefore the hydrogels

15

of chemically cross-linked polyaminophosphazenes present a more advantageous perspective

16

for controlled drug delivery34-37.

17

Chemically cross-linked hydrogels of the polyphosphazenes are rather poorly investigated.

18

There are some data on the radiation cross-linked gels carrying alkyl ether side groups38.

19

Such gels undergo a collapse upon heating. The phase transition temperature lies in a wide

20

interval from 30 to 65 °С depending on chemical structure of the side chains. A remarkable

21

feature of these gels is a high rate of their response to the temperature change. However,

22

practical use of the radiation cross-linked hydrogels is limited by strict safety security

23

requirements for the procedure of manufacturing of these gels. Additionally, gels with alkyl

24

ether side groups can not be assigned to the biodegradable polymers since they are rather

25

stable to the hydrolysis in aqueous medium. In this aspect the biodegradable


Page 5 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

polyaminophosphazenes seem to be more suitable for preparation of thermoresponsive

2

hydrogels of biomedical applications.

3

Despite a motivated interest to the phase transitions of thermoresponsive polymer

4

hydrogels, the information on the transition mechanisms and energetics is rather scarce39-46.

5

Up to date there are no thermodynamic data on the collapse of the chemically cross-linked

6

polyphosphazene hydrogels. Investigation of the energetics of the volume phase transition of

7

polyphosphazene hydrogels and ligand binding-release phenomena conjugated with it would

8

provide a basis for evaluation and control of their performance in drug delivery.

9

In this paper we present a new type of biodegradable polyphosphazene gels – the

10

hydrogels of the cross-linked polymethoxyethylaminophosphazene (PMOEAP). In the

11

preliminary study we showed that thermoresponsive properties of these gels can be induced

12

by a definite ionic composition of the solvent. Upon heating such hydrogels undergo a highly

13

cooperative transition from the swollen state to the collapsed one. We present data on

14

energetics of this transition obtained by high-sensitivity differential scanning calorimetry and

15

discuss the key interactions responsible for thermoresponsivity of the PMOEAP hydrogels.

16 17 18

EXPERIMENTAL SECTION Gel preparation. PMOEAP networks were synthesized in a dry toluene as described

19

earlier24, 47-48 by exhaustive aminolysis of a linear polydichlorophosphazene (M.W. ~

20

13×106.) with methoxyethylamine in the presence of triethylenetetramine (TETA) as a cross-

21

linker at room temperature. The apparent cross-linking density of networks was varied by

22

adjusting the TETA content ( xc = 3, 6 and 9 mol%). The apparent mean hydrophobicity of

23

the network subchains was changed by addition of 9.4 mol% of extra n-alkyl amines with

24

different length of the hydrophobic tail into the reaction mixture (Scheme 1). The stock

25

samples of hydrogels were prepared after swelling of the PMOEAP networks in deionized 5 ACS Paragon Plus Environment


Page 6 of 39

1

(Milli-Q) water at 4 °С for several weeks. Equilibrium weight concentrations of the gels

2

were determined by their dry residue at 105 °C. NHR1 NHR1 NHR2 N P N P N P Cl P Cl

3 4 5 6 7 8

+ 2 n ( R1-NH2 , R2-NH2 , H2N-R3-NH2 ) N

- 2 n HCl n

NHR1 NH

NHR1

R3 NHR1 NH N P N P N NHR1 NHR2

NHR1 P NHR1

n

Scheme 1. Schematic presentation of the synthesis of the cross-linked polymethoxyethylaminophosphazenes: R1 is the amphiphilic side chain ( CH 3O (CH 2 )2 ); R2 is the hydrophobic side chain ( CH 3 (CH 2 )m , m = 2, 3, 4 ); R3 is the cross-link ( (CH 2CH 2 NH )2 CH 2CH 2 , 3-9 mol%) .

9

High-sensitivity differential scanning calorimetry. A stock suspension of the hydrogel

10

for calorimetric measurements was prepared in deionized water (Milli-Q) with a Potter hand

11

glass homogenizer. An average size of the gel particles was of about 10 µm. The polymer

12

concentration in the stock suspension was determined by dry residue after exhaustive drying

13

of the suspension at 105 °C for a day. The concentration of the stock suspension amounted to

14

10 mg mL-1. The working suspensions in different solvents were prepared by dilution of the

15

stock suspension with a stock buffer solution of a desired pH and ionic composition. The

16

polymer concentration in the working suspension was of 1-3 mg mL-1.

17

Calorimetric measurements were carried out with a differential scanning microcalorimeter

18

DASM-4 (“BIOPRIBOR”, Russia) within the temperature range 10–110 °С at and an excess

19

pressure of 0.25 MPa. Preliminary calorimetric experiments at different heating rates (0.125-

20

2.0 K min-1) have shown that the best precision and reproducibility of the heat capacity

21

measurements were achieved at a heating rate of 1.0 K min-1. Therefore this heating rate was

22

chosen for the main study.

23

The primary data processing and conversion of the apparent heat capacity of PMOEAP

24

into the excess heat capacity function of the phase transition was performed using the

25

NAIRTA 2.0 software (Institute of Organoelement Compounds, Moscow, Russia). The 6 ACS Paragon Plus Environment

Page 7 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

transition baseline was obtained by a spline interpolation of the linear segments of the

2

apparent heat capacity function below and above the transition temperature to the temperature

3

of the peak maximum. The temperature of maximum of the excess heat capacity curve was

4

taken as the transition temperature, Tt . The transition enthalpy, ∆ t h , was determined by

5

integration of the excess heat capacity function. The transition width ∆ tT was calculated as a

6

ratio of the transition enthalpy to the maximum value of the excess heat capacity function.

7

The transition heat capacity increment D t c p was defined as a difference in the apparent

8

partial heat capacities of polymer in the collapsed and swollen states of the gel extrapolated

9

to the transition temperature Tt .

10

Equilibrium swelling measurements. These experiments were performed for all

11

obtained PMOEAP gels at temperatures 22 and 95 °C, that is, before the volume phase

12

transition and after its completion, using 100 mM phosphate buffer with pH 7.1 as a solvent.

13

The gel samples were equilibrated at 22 °C for a week, removed from the buffer and their

14

equilibrium concentrations were determined by drying to constant weight at 105 °C. The

15

weighted swollen gel samples were incubated in the buffer at temperature of 95 °C for 1

16

hour, removed from the buffer and weighted once more. Concentrations of the collapsed gel

17

samples were calculated from differences in weights of the swollen and collapsed samples.

18

The obtained weight concentrations of the gels in the swollen and collapsed states were

19

recalculated to polymer volume fractions using the specific partial volume of PMOEAP v2 =

20

0.738±0.001 cm3 g-1 . This value was determined for a linear PMOEAP (M.W. 230 kDa)

21

prepared by the same protocol as the PMOEAP networks but without TETA. This

22

determination was carried out at 20 °C with an automatic oscillating density meter AD-1

23

(“BIOPRIBOR”, Russia). The volume swelling ratio after completion of the collapse



Page 8 of 39

1

transition was defined as a v = f 2 c / f 2 s where f 2c and f 2s are the polymer volume fractions

2

in the collapse and swollen states of the gel at 95 and 22 °C, respectively.

3 4

RESULTS AND DISCUSSION

5

Swelling properties of PMOEAP hydrogels in neat water. Tables 1 and 2 give

6

information on the chemical structure of the investigated PMOEAP hydrogels as well as

7

swelling characteristics of the gels in water. In general, all PMOEAP networks revealed good

8

swelling in water.

9 10 11

12 13 14 15 16 17 18 19 20

Table 1. Swelling Parameters in Water for PMOEAP Hydrogels with Different CrossLinking Density

a

gel

xc a

φ2m b

qm c

Ncd

G1 G2 G3

3 6 9

0.0137±0.0001 0.0310±0.0002 0.0398±0.0003

72.8±0.3 32.4±2.0 25.1±1.9

1.000±0.004 0.259±0.009 0.170±0.006

xc is the apparent cross-linking density, mol%; b φ2m is the equilibrium volume fraction of

polymer in the gel; c qm = 1/ φ2 m is the equilibrium swelling ratio of the gel. d N c = N c / N c* where N c and N c* are the polymerization degrees of subchains in the given gel and the reference gel G1 , respectively. Table 2. Swelling Parameters in Water for PMOEAP Hydrogels Containing Additional Alkyl Groups of Different Length in the Network a

gel G1 G4 G5 G6

21 22 23 24 25

a

xc b alkyl group 3 3 3 3

no propyl butyl pentyl

φ2m c

qm d

0.0137±0.0001 0.0189±0.0021 0.0330±0.0008 0.0326±0.0029

72.8±0.3 52.9±5.7 30.3±0.7 30.7±2.7

The apparent content of additional alkyl groups in the networks is 9.4 mol%; b xc is the

apparent cross-linking density, mol%; c φ2m is the equilibrium volume fraction of polymer in the gel; d qm = 1/ φ2 m is the equilibrium swelling ratio of the gel.


Page 9 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

The equilibrium swelling ratio of the PMOEAP networks decreased notably with an

2

increase in their apparent cross-linking density in a series of the samples G1 > G 2 > G 3

3

(Table 1). According to the definitions, the cross-linking density and the polymerization

4

degree of network subchains N c , that is a number monomer units between the adjacent

5

network knots, are closely related to each other49. Choosing the gel G1 as a reference system,

6

we used a relative value of the polymerization degree of subchains of the gels N c = N c / N c*

7

where N c and N c* are the polymerization degree of subchains in the given gel and the

8

reference gel G1 , respectively. Then, taking into account the relation between the

9

polymerization degree of the network subchains and the swelling equilibrium ratio of gel49,

10

we expressed the relative polymerization degree of the network subchains as: 5

11

æq ö3 ÷ N c = çç m* ÷ ÷ çèq ÷ ø

(1)

m

12

where qm and qm* are the equilibrium swelling ratios for the given gel and the reference gel

13

G1 , respectively. Values of the relative polymerization degree of subchains of the gels G1 ,

14

G 2 and G 3 are presented in Table 1. Naturally, they show that the polymerization degree of

15

subchains decreases with increasing the cross-linking density. Subsequently, we will need a

16

quantitative relation between the polymerizarion degree of subchains and the cross-linking

17

density of polyphosphazene network. We found it in the form:

18

xc* - xco Nc = xc - xco

19

where xc* = 3 mol% is related to the reference gel G1 , and xco corresponds to a gelation

20

threshold. This relation describes well the experimental dependence N c on xc (the

21

coefficient of determination r 2 = 0.999) at xco = 1.90 mol%.

(2)



1

Page 10 of 39

The equilibrium swelling ratio of the PMOEAP gels containing additional alkyl groups of

2

different length are given in Table 2. There is evidently an inverse correlation between the

3

equilibrium swelling ratio of the PMOEAP gels and the number of carbon atoms in the alkyl

4

group, which is a measure of its apparent hydrophobicity. This tendency seems to agree with

5

the Flory-Rehner equation49:

6

qm5/3 µ 0.5 - c

7

where the Flory-Huggins parameter χ has a sense of the reduced free energy of the polymer-

8

solvent interaction. It shows that the larger the parameter c the equilibrium swelling ratio is

9

smaller. Obviously, the parameter c should increase with increasing the apparent mean

10 11

hydrophobicity of the polymer network. Energetics of the volume phase transition (collapse) of the PMOEAP gels was investigated

12

by high-sensitivity differential scanning calorimetry according to a procedure reported

13

earlier39-41. For this purpose a gel suspension with the particle size of about 10 µm in an

14

aqueous solvent was used. The gel G1 with the lowest cross-linking density and

15

hydrophobicity of subchains was chosen as a reference sample.

16

(3)

Salt effects on thermoresponsivity of PMOEAP hydrogels. Figure 1 demonstrates the

17

excess heat capacity functions of the gel G1 in presence of 10 mM of various salts. It is

18

important to stress that in neat water a thermotropic collapse transition of this gel is

19

practically degenerated, i.e. the gel does not show thermoresponsivity in the absence of extra

20

ions. However, addition of 10 mM of phosphate ions changes radically the gel behavior. A

21

high and rather symmetric heat capacity peak appears at 62 °С that corresponds to the

22

transition of the gel from the swollen state into the collapsed one. The distinct heat capacity

23

peaks are observed also in the presence of 10 mM citrate and carbonate ions. The collapse

24

transition of the gel G1 is essentially degenerated in the case of sulfate, fluoride and chloride


Page 11 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 4 3 2 1 0

cEp, J g-1 K -1 1 2

(a)

cEp, J g-1 K -1

(b)

3 4

20 40 60 80 100 20 40 60 80 100 cEp, J g-1 K -1 c , J g-1 K -1 (c) (d) 5 4 5 7 3 6 8 2 1 0 20 40 60 80 100 20 40 60 80 100 T, oC T, oC E p

1 2 3 4 5

Figure 1. Excess heat capacity functions of the PMOEAP hydrogel G1 in water (1) and in solutions of various salts: (2) 10 mM phosphate buffer, (3) 10 mM citrate buffer, (4) 10 mM carbonate buffer, (5) 10 mM imidazole hydrochloride, (6) 10 mM Na 2SO4 , (7) 10 mM NaF, and (8) 10 mM NaCl. рН 7.1.

6

ions. It is interesting that imidazole hydrochloride induces also the gel collapse however its

7

effect is notably weaker than the effects of phosphate, citrate and carbonate.

8

Effects of phosphate ion concentrations on thermoresponsivity of PMOEAP

9

hydrogels. In order to elucidate the mechanism of the ion effects on the collapse we

10

performed calorimetric measurements for the gel G1 at different concentrations of phosphate

11

ions using 0.5-80 mM phosphate buffers with pH 7.1 (Figure 2). The thermograms

12

demonstrated a broad heat capacity peak of the collapse with a rather small heat effect at low

13

buffer concentrations (Figures 2a and 2b). However they changed drastically with the

14

increasing of the buffer concentration (Figures 2c and 2d). The heat capacity peak became

15

narrow and high that revealed a highly cooperative event.

16

The dependences of the thermodynamic transition parameters of the gel G1 on the

17

phosphate buffer concentration at pH 7.1 are shown in Figure 3. Upon an increase in the

18

buffer concentration the transition temperature gradually decreases, while the enthalpy

19

increases.



cEp, J g-1K -1

(a)

6

cEp, J g-1K -1

Page 12 of 39

(b)

4 2 0 10 30 50 70 90 110 10 30 50 70 90 110 cEp, J g-1K -1 cEp, J g-1K -1 (c) (d) 6 4 2 0

1 2 3

10 30 50 70 90 110 T, oC

10 30 50 70 90 110 T, oC

Figure 2. Excess heat capacity functions of the PMOEAP hydrogel G1 at different concentrations of phosphate buffer, mM: (a) 0.5, (b) 2.5, (c) 5.0, and (d) 80. рН 7.1. Tt , oC

70

(a)

60 50

∆th, J g-1 60

(b)

30 0

∆tT, oC 20

(c)

15 10 5

4 5 6 7 8

0 20 40 60 80 100 CPB, mM

Figure 3. Dependences of the transition temperature (a), enthalpy (b) and width (c) for the PMOEAP hydrogel G1 on the phosphate buffer concentration. рН 7.1.


Page 13 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Note, the transition enthalpy of the PMOEAP gel reaches 60 J g-1 in 100 mM phosphate

2

buffer that exceeds significantly the transition enthalpy of hydrogels of poly-N-

3

isopropylacrylamide that are considered as gold standards of the volume phase transition39, 41.

4

The transition width of the PMOEAP gel decreases substantially, and the maximal

5

changes in this parameter are observed at the relatively small buffer concentrations (below 10

6

mM). Such a low content of phosphate anions is not sufficient for manifestation of the

7

lyotropic effect which is known to influence the solubility of hydrophobic compounds in

8

water at the salt concentrations above 0.1 M50.

9

It could be hypothesized that the changes in the transition parameters of the PMOEAP gel

10

reflect the binding of phosphate anions to the polyphosphazene subchains, and as a

11

consequence, the effect of the anions on hydration of the subchains. However, it should not

12

be considered as a purely electrostatic effect. Some specific hydrogen bonds seem to

13

participate in this reaction.

14

As it was shown above (Figure 1), the collapse of the PMOEAP gel was induced by the

15

salts of weak polybasic acids: sodium phosphate, citrate and carbonate. In the presence of the

16

salts of strong acids, including such efficient salting-out agents as sodium sulfate, chloride

17

and fluoride, the gel does not show thermoresponsive behavior. The anions of the weak acids

18

are able to act as the proton donors with the formation of H-bonds between the neighboring

19

bound anions. As a result of this interaction, the water structure making chemical groups are

20

concentrated within the local hydration shell of polyphosphazene subchain. Thus the last

21

acquires an extended ordered structure which can melt cooperatively upon heating. We

22

assume that the melting of the hydration shell of PMOEAP ordered by the bound anions is

23

apparently a main driving factor of the thermoresponsivity of the PMOEAP hydrogels.

24 25

pH effects on thermoresponsivity of PMOEAP hydrogels. The PMOEAP gel subchains carry positive charges in the backbone which can be adjusted by changing pH. We roughly



Page 14 of 39

1

estimated the apparent ionization degree of PMOEAP subchains

2

over the pH range studied using octaethylaminocyclotetraphosphazene as a model of short

3

sequences of phosphazene units. The ionization of this compound is described by two pK a

4

values ( pK a1 8.7 and pK a 2 6.1) illustrating the anti-cooperative effects in the course of the

5

successive protonation of nitrogen atoms51. The calculations showed that the apparent

6

ionization of PMOEAP seems to change from 0.5 to 0.1 over the pH range from pH 5 to 9.

7

Figure 4 shows the collapse thermograms of the gel G1 at different pH from pH 5.8 to 8.6.

8

Significant changes in the peak position and profile are observed upon increasing pH. The

9

collapse peak shifts gradually to the lower temperatures (Figure 4, curves 1-8), increases in

10

height and becomes more narrow up to pH 6.9 (Figure 4, curve 5), and then broadens. It

11

becomes asymmetric at pH 7.6 and 8.6 because of appearance of an additional minor peak to

12

the right of the major one (Figure 4, curves 7-8) 10 8 6 4 2 0

10 8 6 4 2 0

cEp, J g-1K -1 1 2

10 50 cEp, J g-1K -1

10

13 14 15 16 17 18 19

.

cEp, J g-1K -1

50

90

90

(a) 10 8 3 6 4 4 2 0 130 10 50 cEp, J g-1K -1 (c) 10 8 5 6 6 4 2 0 130 10 50 T, oC

(b)

90

130 (d) 7 8

90

130 T, oC

Figure 4. Excess heat capacity functions of the PMOEAP hydrogel G1 at different рН: 5.8 (1), 6.1 (2), 6.5 (3), 6.7 (4), 6.9 (5), 7.4 (6), 7.6 (7) and 8.6 (8). 100 mM phosphate buffer. The pH-dependences of the transition temperature, enthalpy, entropy and width are shown in Figure 5. The transition temperature decreases with the increase in pH. Noteworthy, the 14 ACS Paragon Plus Environment

Page 15 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

collapse temperature changes by 60 °C within a relatively narrow range of pH 6-8

2

corresponding to the physiological values. The transition enthalpy and entropy pass through

3

maximums at pH 7.1 while the transition width is minimal at this pH. Tt , oC

(a) 80

100

∆th, J g-1

(b)

60

80

40 60

20

40 2.5 2.0 1.5 1.0 0.5 0.0

0 10∆ts, J g-1K -1

(d)

15 10 5 6

4 5 6 7 8

(c)20

∆tT, oC

7

8

9 pH

6

7

8

9 pH

Figure 5. pH-dependences of the transition temperature (a), enthalpy (b), apparent entropy (c) and width (d) of the PMOEAP hydrogel G1 . The apparent entropy was calculated as D t s = D t h / Tt . 100 mM phosphate buffer.

xH PO =0.860 -1 4

2

xHPO =0.140 -2 4

(a)

α = 0.48 ∆GH=0.1 xH PO =0.335 2

-1 4

xHPO =0.665 -2 4

(b)

α = 0.27 ∆GH=0.7 xH PO =0.004 2

-2 4

(c)

α = 0.14 ∆GH=0.7

9 10 11 12 13 14 15 16 17

-1 4

xHPO =0.992

Scheme 2. Simplified scheme of formation of the hydrate structure of PMOEAP gel subchains under action of phosphate anions at different pH: (а) рН~5.0; (b) pH~7.0; (c) pH ~ 9.0. The hydrate structure regions are dotted. The density of hatching displays the ordering degree of the structure. D GH is the geometric factor being a measure of the water structure making properties of ions52. Hydrogen bonds between OH and O- 1 groups are shown by arrows. xH PO- 1 and xHPO- 2 are the apparent molar fractions of H 2 PO-4 1 and HPO-4 2 anions in 2

4

4

the 100 mM phosphate buffer, calculated by the BATE software (http://www.chembuddy.com); a is apparent ionization degree of PMOEAP subchains. 15 ACS Paragon Plus Environment


1

Page 16 of 39

When pH increases over the pH range studied the concentration of singly charged

2

phosphate anions decreases but the concentration of doubly charged phosphate anions

3

increases. The singly charged anions dominate, and the polyphosphazene carries a high

4

positive charge at the relatively low pH values. Under these conditions, we observed the high

5

transition temperature as well as the low transition enthalpy. It follows that the transition

6

entropy is rather low. This reveals a relatively low ordering of water molecules in the hydrate

7

structure of the polyphosphazene. Thus the singly charged phosphate anions order weakly the

8

hydrate shell of the polymer even at a rather high binding of these ions to the

9

polyphosphazene matrix (Scheme 2a). This conclusion seems to agree with a rather small

10

value of the geometric factor D GH for H 2 PO-4 1 anions ( D GH = 0.1)52. This factor is a

11

quantitative measure of the water structure making capacity of ionic solutes. Upon increasing

12

pH till pH ~7, the concentration of singly charged phosphate anions decreases but the

13

concentration of doubly charged phosphate anions increases with a decrease in positive

14

charge of the polyphosphazene. As a result, we found a notable decrease in the transition

15

temperature and a very significant increase in the transition enthalpy. This pattern discloses

16

that the binding of doubly charged phosphate anions leads to a significantly higher ordering

17

of the hydrate structure of the polyphosphazene. Apparently, this is a result of the fact that

18

the water structure making capacity of doubly charged phosphate anions is much higher than

19

that of singly charged phosphate anions. Actually, the geometric factor D GH is equal to 0.7

20

for HPO-4 2 ions52. In addition, there is apparently a motif to organize the HPO-4 2 ions bound

21

to the polyphosphazene into a linear array due to formation of hydrogen bonds OH ×××O- 1

22

between the neighboring bound anions (Scheme 2b). The additional hydrogen bonds should

23

favour merging of hydrate shells of the bound anions with formation of extended and highly

24

ordered hydrate structure of the polyphosphazene. This could explain why we observed the


Page 17 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

highly cooperative transition with the low transition temperature and very high transition

2

enthalpy in the presence of predominantly doubly charged phosphate anions (pH ~ 7.0).

3

The ionization of the polyphosphazene declines at higher pH. The number of positively

4

charged sites on the polymer matrix decreases. The average distance between bound

5

phosphate anions tends to increase. Their hydrate shells could not be overlapped (Scheme

6

2c). Consequently, the cooperativity of the hydrate structure of polyphosphazene is

7

disturbed. This state of the polyphosphazene network is most clearly illustrated by a rather

8

significant broadening of the transition at the high pH (Figure 5d). Disordering of the hydrate

9

structure of polyphosphazene could be displayed as a decrease in the transition enthalpy as

10

well as in the transition entropy. The further decrease in the transition temperature

11

demonstrates overcoming of the enthalpic effect.

12

Thermo- and pH-responsibility of the PMOEAP hydrogels together with their

13

biodegradability and non-toxicity determine a high potential of using these gels in systems of

14

the controlled drug delivery. However, a rather high collapse temperature of the PMOEAP

15

gel G1 seriously hinders a possibility of its biomedical application. In this view a search of

16

the ways to lower the collapse temperature of the polyphosphazene hydrogels is relevant. We

17

synthesized the PMOEAP gels with the different cross-linking density and subchain

18

hydrophobicity and investigated their collapse energetics.

19

Effects of cross-linking density and subchain hydrophobicity on thermoresponsivity

20

of PMOEAP hydrogels. Table 3 lists the thermodynamic parameters of the collapse of the

21

PMOEAP gels G1 , G 2 and G 3 with the apparent cross-linking density xc = 3, 6 and 9

22

mol% in 100 mM phosphate buffer of pH 7.1. The increase in the cross-linking density leads

23

to the substantial decrease in the extent of the collapse. The volume swelling ratio after

24

completion of the transition changed from 0.08 at xc = 3 mol% to 0.36 at xc = 9 mol%. The

25

transition temperature changed very slightly with the increase in the cross-linking density. On 17 ACS Paragon Plus Environment


Page 18 of 39

1

the other hand, the transition enthalpy decreased markedly with the increased cross-linking

2

density. The networks with the higher cross-linking density were characterized by larger

3

values of the transition width, i.e. the collapse of the dense hydrogels was less cooperative.

4

Additionally, note that the transition heat capacity increment is negative for all gels, that is,

5

the transition was accompanied by a decrease in the partial heat capacity of the polymer.

6

This feature is typical of aqueous solutions and gels of thermoresponsive polymers39, 41, 53-55.

7

It reveals that hydrophobic hydration of these polymers diminishes significantly upon heating

8

in the course of the conformational or phase transitions.

9 10 11

Table 3. Thermodynamic Parameters of the Volume Phase Transition of PMOEAP Hydrogels with Different Cross-Linking Density a gel

12 13 14 15 16 17 18

xc b 10f 2 s c

10f 2 c d

αve

Tt , °C

G1 3 0.43±0.01 5.31±0.20 0.081±0.005 59.9±0.2 G2 6 0.55±0.04 2.37±0.22 0.232±0.039 59.3±0.3 G3 9 0.54±0.01 1.52±0.03 0.357±0.013 58.8±0.3 a

∆ t h , J g-1

∆Tt , °C

∆ t c p , J g-1 K-1

60.0±1.4 46.9±0.2 39.4±0.3

6.6±0.1 10.3±0.3 10.7±0.3

-0.69±0.04 -0.36±0.06 -0.16±0.06

100 mM phosphate buffer, pH 7.1. b xc is the apparent cross-linking density, mol%; c f 2s is

the volume fraction of polymer in the swollen gel at 22 °C; d f 2c is the volume fraction of polymer in the collapsed gel at 95 °C; e α v = φ2 s / φ2 c is the volume swelling ratio after the transition completion. D t c p is the transition heat capacity increment. Thus, the collapse temperature of the polyphosphazene gels cannot be effectively shifted

19

by variations in the cross-linking density. However, we came to an important conclusion that

20

it would be rational to reduce the cross-linker density of PMOEAP hydrogels within the

21

limits providing the necessary gel strength in order to achieve the maximum extent of the gel

22

collapse.

23

It is well-known that the shift of the hydrophobic-hydrophilic balance of a

24

thermoresponsive polymer to higher hydrophobicity causes a decrease in the phase separation

25

temperature of its aqueous solutions56. We investigated energetics of the thermotropic

26

collapse of the PMOEAP hydrogels with different apparent hydrophobicity of the subchains.

27

An additional hydrophobic substituent was introduced into the structure of the PMOEAP 18 ACS Paragon Plus Environment

Page 19 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

networks G 4 , G5 and G 6 : propyl, butyl and pentyl amine, respectively. Table 4 lists the

2

transition parameters of these gels in comparison with the reference gel G1 which does not

3

contain the additional amines.

4 5 6

Table 4. Thermodynamic Parameters of the Volume Phase Transition of PMOEAP Hydrogels Containing Additional Alkyl Groups of Different Length in the Network a gel G1 G4 G5 G6

7 8 9 10 11 12 13 14 15

alkyl groupb no propyl butyl pentyl

10f 2 s c

10f 2 c d

αve

Tt , °C

∆ t h , J g-1

∆Tt , °C

∆ t c p , J g-1 K-1

0.43±0.01 0.33±0.03 0.47±0.02 0.56±0.02

5.31±0.20 1.70±0.43 2.03±0.06 1.85±0.17

0.081±0.005 0.197±0.066 0.232±0.016 0.304±0.037

59.9±0.2 51.1±0.3 47.2±0.4 42.9±0.4

60.0±1.4 40.7±1.3 23.3±1.9 20.5±1.7

6.6±0.1 5.4±0.2 7.6±0.1 10.5±0.4

-0.69±0.04 -0.9±0.2f

a

100 mM phosphate buffer, pH 7.1. b The apparent content of additional alkyl groups in the networks is 9.4 mol%; c f 2s is the volume fraction of polymer in the swollen gel at 22°C; d f 2c is the volume fraction of polymer in the collapsed gel at 95°C; e α v = φ2 s / φ2 s is the volume swelling ratio after the transition completion. D t c p is the transition heat capacity increment. f

Average values D t c p for gels G4, G5 and G6. The apparent cross-linking density of all gels

is 3 mol%. It is evident that the transition temperature significantly decreases (by almost 20 °C) in a

16

series of the samples G1 > G 4 > G5 > G 6 approaching the physiologic temperature range.

17

The transition enthalpy also decreases, but still retains rather high values (∼20 J g-1). The

18

transition width increases with the increased subchain hydrophobicity, i.e the collapse

19

becomes less cooperative. The transition heat capacity increments of the samples G 4 , G5

20

and G 6 , remaining negative, do not reveal any significant difference, consequently their

21

average value is presented in Table 4. Thus, the necessary shift of the collapse temperature of

22

the polyphosphazene gels seems to be managed by proper adjusting the subchain

23

hydrophobicity.

24

A theoretical generalization of the qualitative considerations on the mechanism of salt-

25

induced thermoresponsivity of PMOEAP hydrogels, given above, is still impossible due to

26

the lack of experimental data on binding of anions of polyfunctional acids to the

27

polyphosphazene chains. Nevertheless, the theoretical analysis of the effects of cross-linking



1

density and apparent hydrophobicity of subchains of the PMOEAP network on its

2

thermoresponsivity could be performed in terms of the classical theory of volume phase

3

transition of polymer gels.

Page 20 of 39

4

Simulations of thermoresponsivity of PMOEAP hydrogels in terms of the classic

5

theory of volume phase transition of polymer gels. According to this theory the equation

6

of transition is of a form57:

7

1 c= ´ f2

é ïì 0 ïï V1 ´ f 2 ´ êf 2 í ïï v2 M 0 N c ê 2f 0 ê ë 2 ïî

1/3 ùïü æ ö ÷ - f - ln(1- f )úïï ççf 2 ÷ ý 2 2 ú çèf 0 ÷ ÷ ï ú 2 ø ûïïþ

(4)

8

where c is the Flory-Huggins interaction parameter, f 2 is the volume fraction of polymer in

9

the gel, V1 is the partial molar volume of the solvent, v2 is the partial specific volume of the

10

polymer, M 0 is the molecular weight of monomer subchain units of the gel, and N c is the

11

polymerization degree of subchains. The volume fraction of polymer f 20 relates to the

12

reference state of gel with strictly Gaussian subchains. In a rough approximation, it can be

13

supposed58 that f 20 ~ 1/ N c 0.5 . In the case of PMOEAP hydrogels V1 = 18 cm3 mol-1 , v2 =

14

0.738 cm3 g-1 and M 0 = 193.

15 16 17

Taking into account a definition of the Flory-Huggins interaction parameter59 it can be assumed for aqueous systems that: c=

D G0hyd RT

(5)

18

where D G0hyd is the free energy of hydration of a monomer unit of the polymer. In general,

19

the free energy of hydration could be estimated using contributions of constituent functional

20

groups of the monomer units60. The monomer unit of PMOEAP consists of a core

21

[- N = P(NH) 2 - ] and two side methoxyethyl branches [- CH 2 - CH 2 - O - CH 3 ] . The

22

contributions of functional groups of the core are still unknown. Nevertheless, the 20 ACS Paragon Plus Environment

Page 21 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

contributions of the side branches could be easily calculated using a very detailed database on

2

contributions of different fragments of amino acid residues of proteins60.

3

The free energy of hydration for a methoxyethyl (MOET) branch can be determined as hyd hyd hyd D GMOET = D GCH + 2D GCH + D GOhyd 3 2

4

(6)

5

hyd hyd where D GCH , D GCH and D GOhyd are the free energy of hydration of methyl and methylene 3 2

6

groups, and oxygen atom, respectively. It was shown60 that the free energy of hydration of an

7

) hyd aliphatic group is equal to the product of an universal tabulated function D Gapo (T ) by the

8

accessible surface area of the group ( ASA ), that is

9 10 11

hyd hyd D GCH = D Gˆ apo ASACH 3 3

(7)

hyd hyd D GCH = D Gˆ apo ASACH 2 2

(8)

and

12 13

It is evident that ASACH 3 = ASAALA = 67 Å2 and ASACH 2 = ASALEU - ASAVAL = 20 Å2

14

where ASAALA , ASALEU and ASAVAL are the tabulated values for apolar fragments of alanine,

15

leucine and valine61. The contribution of oxygen D GOhyd can be determined as a difference

16

between the free energies of hydration of polar fragments of residues of glutamine acid

17

( OOH ) and serine ( OH ):

18

hyd hyd D GOhyd = D GGLU - D GSER

(9)

19

hyd hyd and D GSER The free energies D GGLU are expressed in terms of the tabulated functions of the

20

hyd hyd polar fragments of residues of glutamine acid and serine, D Gˆ GLU (T ) and D Gˆ SER (T ) 60, and

21

their accessible surface areas61, ASAGLU =77 Å2 and ASASER =36 Å2:

22

hyd hyd D GOhyd = D Gˆ GLU ASAGLU - D Gˆ SER ASASER

(10)



1

Page 22 of 39

hyd Thus, suggesting that D G0hyd ; 2D GMOET we can calculate the interaction parameter c for

2

PMOEAP hydrogels as a function of temperature. Using this function it is rather easy to solve

3

eq (1) relative to the volume fraction of polymer f 2 and obtain the collapse transition curve

4

f 2 (T ) . Knowing this dependence we can calculate the heat capacity of hydrogel as a

5

function of temperature.

6

According to the classic theory of polymer gel collapse57, the enthalpy of polymer

7

hydrogel depends on the enthalpy of hydration of monomer unit D H 0hyd , a number of moles

8

of the solvent n1 and the volume fraction of polymer f 2 in the gel:

9

D H = D H 0hyd n1f 2

10

It should be converted to the specific enthalpy of the gel for convenience of comparison with

11

our experimental data:

12 13

D h = D H 0hyd ( v2 / V1 )(1- f 2 )

(11)

(12)

where both D H 0hyd and f 2 are functions of temperature.

14

hyd (T ) could be determined by the same algorithm The dependence D H 0hyd (T ) ; 2D H MOET

15

that was used above for the calculation of the free energy of hydration of MOET monomer

16

hyd hyd hyd unit. The enthalpies of hydration per square Angstrom (Å2) D Hˆ apo at , D Hˆ GLU and D Hˆ SER

17

different temperatures are tabulated in the work62. Finally, taking into account the

18

temperature dependences of the enthalpy of hydration of monomer unit and the volume

19

fraction of polymer ( D H 0hyd (T ) and f 2 (T ) , respectively) we can calculate the heat capacity

20

of the gel as a function of temperature by numeric differentiation of eq (12).

21

First, we will consider effects of the cross-linking density on the collapse of the PMOEAP

22

gels in terms of our model. To do this we need to fix a dependence of the polymerization

23

degree of gel subchains N c on the cross-linking density xc . We accepted this dependence in 22 ACS Paragon Plus Environment

Page 23 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

the following form using the gel with the cross-linking density xc* = 3 mol% as a reference

2

system:

N c = N c*

3

xc* - xco xc - xco

(13)

4

where N c* is the polymerization degree of subchains for the reference gel and the xco

5

corresponds to the gel-point of the system. We assumed that N c* ; 19.8 and xco ; 1.9 mol%.

6

These parameters provided an approximate correspondence between the experimental and

7

calculated values of the volume fraction of polymer for the PMOEAP gels with the cross-

8

linking density xc = 3, 6 and 9 mol% at the initial temperature (22°C). 10-1cp , J g-1 K -1 1 2 3

5 4 3 2 1 0 0

9 10 11 12 13

20

40

60

80

100 o T, C

Figure 6. Simulated heat capacity of the PMOEAP hydrogels with different cross-linking density xc = 3 (1), 6 (2) and 9 (3) mol%. The calculated dependences of heat capacity on temperature for the hydrogels with

14

different cross-linking density are given in Figure 6. In general, they reproduce the main

15

features of the experimental thermograms of the collapse of PMOAP hydrogels. The heat

16

capacity of the gel passes through a maximum in the course of the collapse. Moreover, in full

17

accordance with the experimental data the heat capacity of the gel in the collapsed state is

18

clearly less than that one in the swollen state, that is, D t c p < 0 . 23 ACS Paragon Plus Environment


Page 24 of 39

1

The standard processing of the simulated thermograms of hydrogels with different cross-

2

linking density allowed us to calculate the thermodynamic parameters of collapse of these

3

gels: the transition temperature Tt , the specific transition enthalpy D t h and the transition

4

width D tT . Tt , oC 80

1 2

(a)

60

10-2∆th , J g-1

(b)

4 3 1 2

2 1

40

5 6 7 8 9 10 11 12 13 14

0

3 6 9 (φc)-1 -10-2∆thint , J g-1 (c) 2.8 6 3.0 1 3.2 4 3.4 2 3.6 2 3.8 3 6 9 xc , mol%

3 6 10-1∆tT , oC (d) 3 3 2

9 R∆T t

1

1 1;

0 3

6

2

0

9 xc , mol%

Figure 7. Experimental and simulated thermodynamic parameters of the collapse transition for the PMOEAP hydrogels with the different cross-linking density: (a) experimental (1) and simulated (2) transition temperatures; (b) experimental (1) and simulated dehydration (2) enthalpies of the transition; (c) contribution of internal interactions into the transition enthalpy (1) and reciprocal of the experimental volume fraction of polymer in the collapsed state of the hydrogels (2); (d) experimental (1) and simulated (2) transition widths as well as their ratio (3): RD tT = D tTexp / D tTsim = 0.44±0.03. The experimental and calculated thermodynamic parameters of the collapse transition for

15

the PMOEAP hydrogels against the gel cross-linking density are presented in Figure 7. The

16

model reproduces very well the experimental values of the transition temperature showing

17

that the transition temperature is really independent of the cross-linking density (Figure 7a).

18

The calculated transition enthalpy, that is, in fact the enthalpy of dehydration of polymer

19

network, is far larger than the experimental transition enthalpy. The similar relation is typical

20

of protein denaturation63. It is interpreted by the fact that the denaturation enthalpy is a sum

21

of two contributions opposite in sign, the enthalpy of hydration of amino acid residues and

22

the enthalpy of their various internal cooperative interactions within a dense protein globule. 24 ACS Paragon Plus Environment

Page 25 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Staying within the framework of a rough analogy between the collapse of hydrogel and

2

protein folding, we can approximately estimate energetics of secondary interactions of

3

monomer units of polymer network in the condensed, i.e. collapsed state. By definition the

4

enthalpy of internal interactions D t hint is equal to the difference between the experimental

5

transition enthalpy D t hexp and the enthalpy of hydration D t h hyd :

6 7

D t hint = D t hexp - D t h hyd

(14)

Figure 7c (curve 1) shows that, as expected, the enthalpy of internal interactions is

8

negative. When the cross-linking density increases this enthalpy decreases in absolute value

9

demonstrating weakening of secondary interactions of monomer units in the polymer

10

networks with a larger cross-linking density. It seems obvious that the contribution of

11

secondary interactions of the monomer units of the network should correlate with the specific

12

gel volume, which could be approximately defined as the reciprocal of the gel concentration.

13

The larger contribution of secondary interactions and, correspondingly, a more negative value

14

of the enthalpy of internal interactions should be consistent with the smaller specific volume

15

of the gel, i.e. with a smaller value of the reciprocal of the gel concentration. The dependence

16

of reciprocal of gel concentration in the collapsed state on the cross-linking density is

17

additionally given in Figure 7c (curve 2). The reciprocal of the gel concentration in the

18

collapse state increases with an increase in the cross-linking density. This correlates quite

19

obviously with the increase in the enthalpy of internal interactions. Thus the decrease in the

20

contribution of internal interactions to the transition enthalpy with increasing the cross-

21

linking density is a result of the increase in the average distance between monomer units of

22

the collapsed networks and, as a consequence, weakening their interactions.

23

The calculated and experimental dependences of the transition width on the cross-linking

24

density are symbate. In both cases the transition is broadened with increasing the cross-

25

linking density. This effect finds a simple explanation. The gel network could be considered 25 ACS Paragon Plus Environment


Page 26 of 39

1

as a population of subsystems. Their size is dictated by the polymerization degree of the

2

network subchains. The higher the cross-linking density, the lower the polymerization degree

3

of subchains and, consequently, the smaller is the size of subsystems. On the other hand, it is

4

well known that the smaller the subsystem size, the wider the conformational or phase

5

transition, which the subsystem undergoes64. In other words, it is reasonable to suggest that

6

D tT : 1/ N c as a crude approximation. Then D tTexp / D tTsim = N csim / N cexp where

7

D tTexp , D tTsim , N cexp and N csim are the experimental and simulated values of the transition width

8

and the polymerization degree of subchains, respectively. The curve 3 in Figure 7d shows

9

that the ratio D tTexp / D tTsim =0.44±0.03 independently on the cross-linking density. Thus it

10

becomes clear that the discrepancy in the experimental and calculated values of the transition

11

width is most likely the result of the crude estimation of the polymerization degree of

12

subchains in the reference gel ( N c* ; 19.8).

13

The above algorithm can be extended to the PMOEAP hydrogels containing additionally

14

the alkyl side branches of different lengths. For this it is only necessary to re-determine the

15

hydration free energy and enthalpy of the network monomer unit in the forms:

16 17 18

D G0hyd = (1-

xR x hyd + R 2D GRhyd )2D GMOET 100 100

(15)

D H 0hyd = (1-

xR x hyd )2D H MOET + R 2D H Rhyd 100 100

(16)

and

19

where xR = 9.4 mol% is an apparent content of the additional alkyl side chains in the

20

networks G 4, G5 and G 6 . The thermodynamic characteristics of these chains D GRhyd and

21

D H Rhyd are determined as:

22

hyd hyd D GRhyd = D GCH + nD GCH 3 2

(17)


Page 27 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


and hyd hyd D H Rhyd = D H CH + nD H CH 3 2

2

(18)

3

where n = 2, 3 and 4 for the gels G 4 , G5 and G 6 . The algorithm for calculations of the

4

contributions of methylene and methyl groups was described above. 10-1cp , J g-1 K -1 1 2 3 4

10 8 6 4 2 0 0

5 6 7 8 9 10

20

40

60

80

100 o T, C

Figure 8. Simulated heat capacity of the PMOEAP hydrogels with the cross-linking density xc = 3 mol% not containing (1) and containing 9.4 mol% additional propyl (2), butyl (3) and pentyl (4) groups. Simulated heat capacity functions of the hydrogels G 4, G5 and G 6 are shown in Figure

11

8. They demonstrate a regular shift of the collapse transition peak to lower temperatures with

12

an increase in the length of additional alkyl branches. The simulated and experimental

13

transition parameters for the gels G 4 , G5 and G 6 against the carbon number in the

14

corresponding alkyl branch are compared in Figure 9. Importantly, the model reproduces

15

rather well the transition temperatures of the gels containing the alkyl branches of different

16

length (Figure 9a).

17 18

The calculated transition enthalpy is again much larger than the experimental one (Figure 9b). This reveals a very large contribution of internal interactions into the collapsed state of



Tt , oC 80

1 2

60 40 20 0 1 2 3 -10-2∆thint , J g-1 (c) 8 10 12 14 16 2 18 0 1 2 3

1 2 3 4 5 6 7 8 9 10 11 12

Page 28 of 39

10-2∆th , J g-1

(b) (a) 10 8 6 1 4 2 2 0 4 5 0 1 2 3 4 5 10-1∆tT , oC (φc)-1 (d) 8 1 2 1 2 6 4 1 2 0 4

5 CN

0 0

1

2

3

4

5 CN

Figure 9. Experimental and simulated thermodynamic parameters of the collapse transition for the PMOEAP hydrogels of the constant cross-linking degree ( xc = 3 mol%) containing 9.4 mol% additional alkyl groups with different carbon numbers ( CN ): (a) experimental (1) and simulated (2) transition temperatures; (b) experimental (1) and simulated dehydration (2) enthalpies of the transition; (c) contribution of internal interactions into the transition enthalpy (1) and reciprocal of the experimental polymer volume fraction in the collapsed state of the hydrogels (2); (d) experimental (1) and simulated (2) transition widths. CN = 0 (control gel without additional alkyl groups), CN = 3 (propyl), CN = 4 (butyl), CN = 5 (pentyl). the gels. The enthalpy of internal interactions reduces significantly with increasing of the

13

alkyl branch length. At the same time, there is no clear correlation between this enthalpy and

14

the specific gel volume in the collapsed state ( ~ 1 / f 2 c ) (Figure 9c). From the gel G 4 to the

15

gel G 6 , the enthalpy D hint significantly decreases but the value of 1 / f 2 c remains practically

16

constant. In this regard, it is necessary to note that the enthalpy of internal interactions should

17

be determined not only by an average distance between the alkyl branches in the collapsed

18

state of the gel but also by their cohesion density. We have estimated the apparent cohesion

19

energy for propyl, butyl and pentyl by the group contribution method65. It increases smoothly

20

over the given alkyl array from 222 to 250 J cm-3 demonstrating a strengthening of alkyl-

21

alkyl interactions. Presumably, this effect determines a decrease in the enthalpy D hint passing


Page 29 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

from the gel G 4 ( CN = 3 ) to the gel G 6 ( CN = 5) although the specific volumes of these

2

gels in the collapsed state ( ~ 1 / f 2 c ) are rather comparable.

3

There is a significant discrepancy in the calculated and experimental dependences of the

4

transition width on the alkyl branch length (Figure 9d). According to the simulation the

5

transition length should slightly decrease with the increased length of additional alkyl branch

6

(curve 1). In fact, it increases passing from the gel G 4 ( CN = 3) to the gel G 6 ( CN = 5)

7

(curve 2). Since the transition width correlates directly with the size of the gel network

8

subchains it is possible to suggest that the introduction of additional hydrophobic side chains

9

into the polyphosphazene network produces an increase in its cross-linking density as a result

10 11

of hydrophobic interaction of these side chains. Therefore, the classic theory of volume phase transition of polymer gels coupled with the

12

data on energetics of hydration for different molecular surfaces borrowed from protein

13

science reproduces, in general, correctly thermodynamics of the thermotropic collapse of

14

PMOEAP hydrogels.

15 16

CONCLUSIONS

17

A new thermoresponsive biodegradable polymer system was found – the chemically cross-

18

linked polyaminophosphazene hydrogels. In contrast to other known to date

19

thermoresponsive polymers, these hydrogels acquire thermoresponsivity (thermotropic

20

collapse) only in the presence of the definite anions formed in water as a result of dissociation

21

of the weak polybasic acids. Based on the experimental data we hypothesize that the primary

22

driving force of the collapse is the binding of these anions, capable to be proton donors, to

23

the polyphosphazene backbone. This reaction promotes concentration of the chemical groups

24

making the water structure in the local hydration shell of the gel subchains. The ordered

25

hydration shell melts cooperatively upon heating thus inducing condensation of the subchains 29 ACS Paragon Plus Environment


Page 30 of 39

1

of the PMOEAP gel. The obtained results open wide opportunities for tuning the gel

2

morphology and collapse parameters in accordance with the required tasks. The

3

polyelectrolyte nature of the PMOEAP networks seems to enable its binding affinity for

4

charged ligands, which can be controlled by temperature, pH and ion composition of the

5

solvent. Finally, we showed that the high resolution thermodynamic data of protein science

6

on hydration of various molecular surfaces could be very useful for simulation of the

7

thermoresponsive behaviour of polymers in water environment. However, such simulations

8

demonstrate that the contribution of dehydration of polymer upon the gel collapse determines

9

only a part of the transition enthalpy. The rest predominant part of the transition enthalpy is

10

originated from internal cooperative van der Waals interactions of gel subchains in the

11

collapsed state.

12 13

ACKNOWLEDGEMENTS

14

This work was supported in part by the Russian Foundation for Basic Research under the

15

Project 16-03-00306.

16


Page 31 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

REFERENCES

2

(1)

3 4

isopropylacrylamide) Hydrogel. J. Appl. Polym. Sci. 1994, 52, 85-89. (2)

5 6

(3)

(4)

Macrospheres. J. Microencapsul. 2006, 23, 677-685. (5)

12 (6)

Shibayama, V.; Tanaka, T. Volume Phase Transition and Related Phenomena of Polymer Gels. Adv. Polym. Sci. 1993, 109, 1-62.

(7)

16 17

Schild, H. G. Poly(N-isopropylacrylamide) - Experiment, Theory and Application. Progr. Polym. Sci. 1992, 17, 163-249.

14 15

Lewis, G.; Coughlan, D. C.; Lane, M. E.; Corrigan, O. I. Preparation and Release of Model Drugs from Thermally Sensitive Poly(N-isopropylacrylamide) Based

10

13

Alvarez-Lorenzo, C.; Concheiro, A. Reversible Adsorption by a pH- and Temperature-Sensitive Acrylic Hydrogel. J. Controll. Release 2002, 80, 247-257.

9

11

Serres, A.; Baudys, M.; Kim, S. W. Temperature and pH-Sensitive Polymers for Human Calcitonin Delivery. Pharm. Res. 1996, 13, 196-201.

7 8

Park, T. G.; Hoffman, A. S. Deswelling Characteristics of Poly(N-

Gutowska, A.; Bark, J. S.; Kwon, I. C.; Bae, Y. H.; Cha, Y.; Kim, S. W. Squeezing Hydrogels for Controlled Oral Drug Delivery. J. Controll. Release 1997, 48, 141-148.

(8)

Song, H. J.; Hong, N. Y.; Kim, K. H.; Shin, Y. J.; Lee, J. O. Study on Synthesis of

18

Ionized N-isopropylacrylamide Hydrogel and Its pH- and Thermo-Sensitive Phase

19

Transition Behavior. Polym.-Korea 1997, 21, 999-1005.

20

(9)

Vakkalanka, S. K.; Brazel, C. S.; Peppas, N. A. Temperature- and pH-Sensitive

21

Terpolymers for Modulated Delivery of Streptokinase. J. Biomater. Sci. Polym. Ed.

22

1996, 8, 119-129.

23 24

(10)

Hoffman, A. S. Bioconjugates of Intelligent Polymers and Recognition Proteins for Use in Diagnostics and Affinity Separations. Clin. Chem. 2000, 46, 1478-1486.



1

(11)

Lee, W. F.; Chiu, R. J. Thermoreversible Hydrogel. XVII. Investigation of the Drug

2

Release Behavior for [N-isopropylacrylamide-co-Trimethyl Acrylamidopropyl

3

Ammonium Iodide-co-3-Dimethyl (Methacryloyloxyethyl) Ammonium Propane

4

Sulfonate] Copolymeric Hydrogels. J. Appl. Polym. Sci. 2002, 86, 1592-1598.

5

(12)

Page 32 of 39

Yin, X.; Hoffman, A. S.; Stayton, P. S. Poly(N-isopropylacrylamide-co-Propylacrylic

6

Acid) Copolymers that Respond Sharply to Temperature and pH. Biomacromolecules

7

2006, 7, 1381-1385.

8

(13)

9

Property of PNIPAAM-Based Copolymer Hydrogels. J. Mater. Sci.-Mater. M 2007,

10 11

Zhang, X. Z.; Chu, C. C. Influence of Polyelectrolyte on the Thermosensitive

18, 1771-1779. (14)

Petrusic, S.; Jovancic, P.; Lewandowski, M.; Giraud, S.; Grujic, S.; Ostojic, S.;

12

Bugarski, B.; Koncar, V. Properties and Drug Release Profile of Poly(N-

13

isopropylacrylamide) Microgels Functionalized with Maleic Anhydride and Alginate.

14

J. Mater. Sci. 2013, 48, 7935-7948.

15

(15)

16 17

Drug Release. J. Mater. Chem. B 2013, 1, 3464-3469. (16)

18 19

(17)

24

Hoffman, A. S. Hydrogels for Biomedical Applications. Adv. Drug Deliver. Rev. 2012, 64, 18-23.

(18)

22 23

Hoffman, A. S. The Origins and Evolution of "Controlled" Drug Delivery Systems. J. Controll. Release 2008, 132, 153-163.

20 21

Qian, J.; Wu, F. P. Thermosensitive PNIPAM Semi-Hollow Spheres for Controlled

Qiu, Y.; Park, K. Environment-Sensitive Hydrogels for Drug Delivery. Adv. Drug Deliver. Rev. 2012, 64, 49-60.

(19)

Hoffman, A. S. Stimuli-Responsive Polymers: Biomedical Applications and Challenges for Clinical Translation. Adv. Drug Deliver. Rev. 2013, 65, 10-16.


Page 33 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


(20)

2 3

Lee, B. H.; Lee, Y. M.; Sohn, Y. S.; Song, S. C. Thermosensitive and HydrolysisSensitive Poly(organophosphazenes). Polym. Int. 2002, 51, 658-660.

(21)

Lee, B. H.; Song, S. C. Synthesis and Characterization of Biodegradable

4

Thermosensitive Poly(organophosphazene) Gels. Macromolecules 2004, 37, 4533-

5

4537.

6

(22)

Lee, S. B.; Song, S. C.; Jin, J. I.; Sohn, Y. S. A New Class of Biodegradable

7

Thermosensitive Polymers. 2. Hydrolytic Properties and Salt Effect on the Lower

8

Critical Solution Temperature of Poly(organophosphazenes) with

9

Methoxypoly(ethylene glycol) and Amino Acid Esters as Side Groups.

10 11

Macromolecules 1999, 32, 7820-7827. (23)

Lee, B. H.; Lee, Y. M.; Sohn, Y. S.; Song, S. C. Synthesis and Characterization of

12

Thermosensitive Poly(organophosphazenes) with Methoxypoly(ethylene glycol) and

13

Alkylamines as Side Groups. B. Kor. Chem. Soc. 2002, 23, 549-554.

14

(24)

Grinberg, V. Y.; Grinberg, N. V.; Burova, T. V.; Dubovik, A. S.; Tur, D. R.; Papkov,

15

V. S. Phase Separation in Aqueous Solutions of Polyethylaminophosphazene

16

Hydrochloride During Heating. Polym. Sci. Ser. A 2010, 52, 1220-1227.

17

(25)

Song, S. C.; Lee, S. B.; Jin, J. I.; Sohn, Y. S. A New Class of Biodegradable

18

Thermosensitive Polymers. I. Synthesis and Characterization of

19

Poly(organophosphazenes) with Methoxy-poly(ethylene glycol) and Amino Acid

20

Esters as Side Groups. Macromolecules 1999, 32, 2188-2193.

21

(26)

Kang, G. D.; Cheon, S. H.; Khang, G.; Song, S. C. Thermosensitive

22

Poly(organophosphazene) Hydrogels for a Controlled Drug Delivery. Eur. J. Pharm.

23

Biopharm. 2006, 63, 340-346.



1

(27)

Page 34 of 39

Kang, G. D.; Cheon, S. H.; Song, S. C. Controlled Release of Doxorubicin from

2

Thermosensitive Poly(organophosphazene) Hydrogels. Int. J. Pharm. 2006, 319, 29-

3

36.

4

(28)

Cho, J. K.; Hong, K. Y.; Park, J. W.; Yang, H. K.; Song, S. C. Injectable Delivery

5

System of 2-Methoxyestradiol for Breast Cancer Therapy Using Biodegradable

6

Thermosensitive Poly(organophosphazene) Hydrogel. J. Drug Target. 2011, 19, 270-

7

280.

8

(29)

9 10

Poly(organophosphazene) Gel. Macromolecules 2002, 35, 3876-3879. (30)

11 12

(31)

Ahn, S.; Ahn, S. W.; Song, S. C. Thermosensitive Amphiphilic Polyphosphazenes and Their Interaction with Ionic Surfactants. Colloids Surf. A 2008, 330, 184-192.

(32)

15 16

Loh, X. J.; Li, J. Biodegradable Thermosensitive Copolymer Hydrogels for Drug Delivery. Expert Opin. Ther. Pat. 2007, 17, 965-977.

13 14

Lee, B. H.; Lee, Y. M.; Sohn, Y. S.; Song, S. C. A Thermosensitive

Joo, M. K.; Park, M. H.; Choi, B. G.; Jeong, B. Reverse Thermogelling Biodegradable Polymer Aqueous Solutions. J. Mater. Chem. B 2009, 19, 5891-5905.

(33)

Lee, S. M.; Chun, C. J.; Heo, J. Y.; Song, S. C. Injectable and Thermosensitive

17

Poly(organophosphazene) Hydrogels for a 5-Fluorouracil Delivery. J. Appl. Polym.

18

Sci. 2009, 113, 3831-3839.

19

(34)

Potta, T.; Chun, C.; Song, S. C. Chemically Crosslinkable Thermosensitive

20

Polyphosphazene Gels as Injectable Materials for Biomedical Applications.

21

Biomaterials 2009, 30, 6178-6192.

22

(35)

23 24 25

Potta, T.; Chun, C.; Song, S. C. Injectable, Dual Cross-Linkable Polyphosphazene Blend Hydrogels. Biomaterials 2010, 31, 8107-8120.

(36)

Potta, T.; Chun, C.; Song, S. C. Controlling the Degradation Rate of Thermoresponsive Photo-Cross-Linked Poly(organophosphazene) Hydrogels with


Page 35 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Compositions of Depsipeptide and Peg Chain Lengths. Polym. Degrad. Stabil. 2011,

2

96, 1261-1270.

3

(37)

Potta, T.; Chun, C.; Song, S. C. Design of Polyphosphazene Hydrogels with Improved

4

Structural Properties by Use of Star-Shaped Multithiol Crosslinkers. Macromol.

5

Biosci. 2011, 11, 689-699.

6

(38)

7 8

Allcock, H. R.; Ambrosio, A. Synthesis and Characterization of pH-Responsive Poly(organophosphazene) Hydrogels. ACS Sym. Ser. 2003, 833, 82-101.

(39)

9

Grinberg, N. V.; Dubovik, A. S.; Grinberg, V. Y.; Kuznetsov, D. V.; Makhaeva, E. E.; Grosberg, A. Y.; Tanaka, T. Studies of the Thermal Volume Transition of Poly(N-

10

isopropylacrylamide) Hydrogels by High-Sensitivity Differential Scanning

11

Microcalorimetry. 1. Dynamic Effects. Macromolecules 1999, 32, 1471-1475.

12

(40)

Grinberg, V. Y.; Grinberg, N. V.; Mikheeva, L. M.; Dembo, A. T.; Makhaeva, E. E.;

13

Khokhlov, A. R.; Grosberg, A. Y.; Tanaka, T. A New Hydrogel System Undergoing a

14

Volume Phase Transition Upon Heating. Macromol. Chem. Phys. 1999, 200, 1603-

15

1607.

16

(41)

Grinberg, V. Y.; Dubovik, A. S.; Kuznetsov, D. V.; Grinberg, N. V.; Grosberg, A. Y.;

17

Tanaka, T. Studies of the Thermal Volume Transition of Poly(N-

18

isopropylacrylamide) Hydrogels by High-Sensitivity Differential Scanning

19

Microcalorimetry. 2. Thermodynamic Functions. Macromolecules 2000, 33, 8685-

20

8692.

21

(42)

Kawasaki, H.; Sasaki, S.; Maeda, H.; Nishinari, K. Effect of the Introduced Charge on

22

the Thermal Behavior of N-isopropylacrylamide Gels in Water and NaCl Solutions.

23

Langmuir 2000, 16, 3195-3199.

24 25

(43)

Burova, T. V.; Grinberg, N. V.; Dubovik, A. S.; Tanaka, K.; Grinberg, V. Y.; Grosberg, A. Y. Effects of Ligand Binding on Relative Stability of Subchain



1

Conformations of Weakly Charged N-isopropylacrylamide Gels in Swollen and

2

Shrunken States. Macromolecules 2003, 36, 9115-9121.

3

(44)

Page 36 of 39

Ge, H.; Ding, Y. W.; Ma, C. C.; Zhang, G. Z. Temperature-Controlled Release of

4

Diols from N-isopropylacrylamide-co-Acrylamidophenylboronic Acid Microgels. J.

5

Phys. Chem. B 2006, 110, 20635-20639.

6

(45)

7 8

Kano, M.; Kokufuta, E. On the Temperature-Responsive Polymers and Gels Based on N-Propylacrylamides and N-Propylmethacrylamides. Langmuir 2009, 25, 8649-8655.

(46)

9

Burova, T. V.; Grinberg, N. V.; Kalinina, E. V.; Ivanov, R. V.; Lozinsky, V. I.; Alvarez-Lorenzo, C.; Grinberg, V. Y. Thermoresponsive Copolymer Cryogel

10

Possessing Molecular Memory: Synthesis, Energetics of Collapse and Interaction with

11

Ligands. Macromol. Chem. Phys. 2011, 212, 72-80.

12

(47)

Tur, D. R.; Pergushov, D. V.; Babin, I. A.; Papkov, V. S.; Zezin, A. B. New

13

Heteroorganic Polyelectrolytes: Protonated Polyalkylaminophosphazenes. Polym. Sci.

14

Ser. B 2009, 51, 1704-1711.

15

(48)

Grinberg, V. Y.; Burova, T. V.; Grinberg, N. V.; Dubovik, A. S.; Tur, D. R.; Usov, A.

16

I.; Papkov, V. S.; Khokhlov, A. R. Conformational Energetics of Interpolyelectrolyte

17

Complexation between ι-Carrageenan and Poly(methylaminophosphazene) Measured

18

by High-Sensitivity Differential Scanning Calorimetry. Langmuir 2011, 27, 7714-

19

7721.

20

(49)

21 22 23

Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, New York, 1953. pp 577-581.

(50)

Zhang, Y.; Cremer, P. S. Interactions between Macromolecules and Ions: The Hofmeister Series. Curr. Opin. Chem. Biol. 2006, 10, 658-663.


Page 37 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


(51)

Feakins, D.; Last, W. A.; Shaw, R. A. Structure and Basicity. II. The Basicity of Fully

2

Aminolyzed Cyclotriphosphazatrienes and Cyclotetraphosphazatetraenes in

3

Nitrobenzene and Water. J. Chem. Soc. 1964, 4464-4471.

4

(52)

5 6

Marcus, Y. Effect of Ions on the Structure of Water: Structure Making and Breaking. Chem. Rev. 2009, 109, 1346-1370.

(53)

Dubovik, A. S.; Makhaeva, E. E.; Grinberg, V. Y.; Khokhlov, A. R. Energetics of

7

Cooperative Transitions of N-vinylcaprolactam Polymers in Aqueous Solutions.

8

Macromol. Chem. Phys. 2005, 206, 915-928.

9

(54)

Dubovik, A. S.; Grinberg, N. V.; Grinberg, V. Y. Energetics of Phase Separation in

10

Aqueous Solutions of Poly(N-isopropylacrylamide). Polym. Sci. Ser. A 2010, 52, 565-

11

573.

12

(55)

Grinberg, V. Y.; Burova, T. V.; Grinberg, N. V.; Dubovik, A. S.; Plaschina, I. G.;

13

Laptinskaya, T. V.; Xiong, Y. B.; Yao, P.; Khokhlov, A. R. Energetics and

14

Mechanism of Conformational Transitions of Protein-Like NIPAM-Sodium Styrene

15

Sulfonate Copolymers in Aqueous Solutions. Macromol. Chem. Phys. 2015, 216,

16

2344-2355.

17

(56)

Taylor, L. D.; Cerancowski, L. D. Preparation of Films Exhibiting a Balanced

18

Temperature Dependence to Permeation by Aqueous Solutions. A Study of Lower

19

Consolute Behavior. J. Polym.Sci. Part A 1975, 13 2551-2570.

20

(57)

21 22

Volume Phase Transition. Phase Transit. 1994, 47, 183-240. (58)

23 24 25

Hirotsu, S. Static and Time-Dependent Properties of Polymer Gels around the

Khokhlov, A. R. Swelling and Collapse of Polymer Networks. Polymer 1980, 21, 376-380.

(59)

Flory, P. J. Principle of Polymer Chemistry; Cornell University Press: Ithaca, New York, 1953. pp 510-511.



1

(60)

Page 38 of 39

Privalov, P. L.; Makhatadze, G. I. Contribution of Hydration to Protein Folding

2

Thermodynamics. 2. The Entropy and Gibbs Energy of Hydration. J. Mol. Biol. 1993,

3

232, 660-679.

4

(61)

5 6

Proteins. J. Mol. Biol. 1987, 196, 641-656. (62)

7 8

(63)

13

Makhatadze, G. I.; Privalov, P. L. Energetics of Protein Structure. Adv. Protein Chem. 1995, 47, 307-425.

(64)

11 12

Makhatadze, G. I.; Privalov, P. L. Contribution of Hydration to Protein Folding Thermodynamics. 1. The Enthalpy of Hydration. J. Mol. Biol. 1993, 232, 639-659.

9 10

Miller, S.; Janin, J.; Lesk, A. M.; Chothia, C. Interior and Surface of Monomeric

Hill, T. L. Thermodynamics of Small Systems. W.A. Benjamin, Inc.: New York, 1963, p 89.

(65)

Fedors, R. F. A Method for Estimating Both the Solubility Parameters and Molar Volume of Liquids. Polym. Eng. Sci. 1974, 14, 147-154.

14 15


Page 39 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

TOC Graphic

Cp

Collapse of PMOEAP hydrogel is induced by H-donor anions

SALT-FREE SO-2 4 HPO-2 4 T, oC

3

40

60

80

100


Salt-Induced Thermoresponsivity of Cross-Linked

Recommend Documents