pH and Temperature Sensitive Block Copolymer Hydrogels Based on

Chapter 7

Downloaded by OHIO STATE UNIV LIBRARIES on July 1, 2012 | http://pubs.acs.org Publication Date: November 7, 2008 | doi: 10.1021/bk-2008-0992.ch007

pH and Temperature Sensitive Block Copolymer Hydrogels Based on Poly(ethylene glycol) and Sulfamethazine Doo Sung Lee, Woo Sun Shim, and Dai Phu Huynh Department of Polymer Science and Engineering, SungKyunKwan University, Suwon, Gyeonggi 440-746, Korea

A novel p H and temperature sensitive block copolymer was prepared by adding a p H sensitive moiety to a temperature sensitive block copolymer. A n oligomer incorporating pH-sensitive moieties and containing an s-triazine ring was synthesized from sulfamethazine (SM) by radical polymerization. Novel p H - and temperature-sensitive biodegradable penta block copolymers were synthesized from the sulfamethazine oligomer (OSM) incorporating pH-sensitive moieties and a temperature sensitive degradable triblock copolymer hydrogel style ABA based on P E G . The sol-gel phase transition behavior of these block copolymers was investigated both in solution and injection into PBS buffer at p H 7.4 and 37°C. Aqueous solutions of these block copolymers showed sol-gel transition behavior upon both temperature and p H changes under physiological conditions (37°C, p H 7.4). When the block copolymer solution is in the sol state at 10°C and p H 8.0, both temperature and p H changes are needed for gelation to occur. The sol-gel transition properties of these block copolymers are influenced by the hydrophobic/hydrophilic balance of the copolymers, block length, hydrophobicity, stereoregularity of the hydrophobic components within the block copolymer, and the ionization of the p H functional groups in the copolymer, which depends on © 2008 American Chemical Society

107

In New Delivery Systems for Controlled Drug Release from Naturally Occurring Materials; Parris, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


108 the environmental pH. The degradation time of these polymers could be controlled by changing the hydrophobic block of the temperature sensitive degradable triblock copolymer hydrogel. These materials could be employed as injectable carriers for hydrophobic drugs and proteins, etc. Gelation inside the needle can be prevented by increasing the temperature during the injection of the hydrogel, because it does not change into the gel form solely upon increasing the temperature. This material could even be used for a long guide catheter into the body.

Hydrogels that exhibit both liquid-like and solid-like behavior exhibit a wide variety of functional properties (i.e. swelling, mechanical, permeation, surface and optical), and these have provided many potential applications for hydrogels in fields such as medicine, agriculture, and biotechnology (1-4). Stimuli-sensitive hydrogels have attracted considerable attention as intelligent materials in the fields of biochemistry and biomedicine, due to their ability to detect environmental changes and undergo structural changes by themselves, such as changes in their solubility and swelling ratio (5-8). Various stimulisensitive hydrogels that respond to pH (9), temperature (10, 11), electric fields (12, 13), and other stimuli have been studied both experimentally and theoretically (14, 15). Among the stimuli-sensitive materials that have been developed so far, polymers showing a sol-to-gel transition with changing temperature have been proposed for use as injectable drug delivery systems (16-18). For example, temperature responsive hydrogels (e.g., Pluronics (BASF), Poloxamers (ICI) (19) and block copolymers composed of poly(ethylene oxide) (PEO) and polypropylene oxide (PPO) (20) have been studied by many researchers. In aqueous solutions, these polymers undergo a temperature induced reversible sol-gel transition upon heating and cooling. Neither Pluronics nor Poloxamers are considered an optimal system for the delivery of drugs, because they are nonbiodegradable. Therefore, biodegradable thermo-reversible hydrogels have been studied as controlled release drug carriers, because of their nontoxicity and biocompatibility. Block copolymers composed of P E G (poly(ethylene glycol)) and P L A (poly(lactic acid)) [or P L G A (poly(lactic acidco-glycolic acid))] (21) chitosan derivatives (22), polyphosphazene (23) methylcellulose (24), etc., have been proposed as biodegradable thermo-sensitive hydrogels. Among these biodegradable thermo-sensitive hydrogels, P L G A P E G P L G A hydrogel has been used as an injectable drug delivery system, owing to its long-term persistence in the gel form (25). However, these hydrogels have several unresolved drawbacks, which limit their application in injectable drug delivery systems. When temperature-sensitive hydrogels are injected into the



109 body with a syringe, they tend to change into a gel as the needle becomes warmed by the body temperature. This change makes it difficult to inject them into the body. Also, after being injected, these hydrogels tend to undergo rapid degradation of the block copolymer, which consequently produces the acidic monomers such as lactic acid or glycolic acid. Since the low-pH environment of the hydrogel caused by the acidic monomer is known to be deleterious to some proteins and nucleic acids, the p H change which occurs within these biodegradable hydrogels is an important consideration (26). K i m et al. used two crosslinked copolymers, poly(methacrylic acid-comethacryloxyethyl glucoside) [P(MMA-co-MEG)] and poly(methacrylic acid-gethylene glycol) [P(MMA-g-EG)], to determine the mechanism of penetrant transport through anionic p H sensitive hydrogels (27). They observed that the water transport mechanism was significantly dependent on the p H of the swelling medium. A t high p H (higher than the pKa of the gel), the water transport was controlled more by polymer relaxation than by penetrant diffusion. For both P ( M M A - c o - M E G ) and P(MMA-g-EG) hydrogels, the swelling mechanism exhibited little dependence on the copolymer compositions of the hydrogels at the same pH. As such, the characteristics of these systems for drug delivery applications were investigated (28), and it was found that the mesh size of these hydrogels changed from small (18-35 Â) in the collapsed state at p H 2.2 to very large (70-111 Â) at p H 7.0, and increased between two and six times during the swelling process, demonstrating some potential disadvantages for use as drug delivery systems by the subcutaneous injection method. Herein, we report investigations of novel p H and temperature sensitive block copolymer hydrogels based on polyethylene glycol (PEG) and sulfamethazine (29, 30). Sulfamethazine oligomer (OSM) was used as a pH-sensitive moiety. The two temperature sensitive block copolymers used were an A B A type block copolymer composed of poly(E-caprolactone-co-lactide) (PCLA) and P E G ( P C L A - P E G P C L A ) and block copolymer polyethylene glycol-poly (glycolide-co-εcaprolactone) (PCGA-PEG-PCGA), which have different degradation times depending on P C L A and P C G A , respectively. The objectives of this research were to study the effects of O S M , P C L A , P C G A , and P E G on the sol-gel phase transition of these pH/temperature sensitive penta-block hydrogels, to study the degradation of these block copolymers, and to study drug loading into polymer solutions and release out of the hydrogels of these copolymers.

Experimental Details Materials and Methods Poly(ethylene glycol) (PEG) was purchased from Sigma-Aldrich (St. Louis, M O ) (Mn=1000, 1500 and 2000) and ID Biochem, Inc. (Seoul, Korea)


no (Mn=1750), and recrystallized in n-hexane and dried in vacuum for 3 days prior to use. D,L-lactide (LA), ε-caprolactone (CL), stannous octoate [Sn(Oct) ] were obtained from Sigma and dried for 24 hours under vacuum at ambient temperature prior to use. N , N -dimethyl formamide (DMF) anhydrous and glycolide (GA) were obtained from Polyscience Boehringer Ingelheimm. Methylene chloride (anhydrous), methacryloyl chloride, 3-mercaptopropionic acid (MPA), dicyclohexyl carboimide (DCC), and 4-dimethyl amino pyridine ( D M A P ) were used as received from Aldrich, whereas, sulfamethazine and 2, Tazobisisobutyronitrile (AIBN) were supplied by Sigma and Junsei Co., respectively. A I B N was recrystallized from methanol twice prior to use. Sulfamethazine was obtained from Sigma and used as received. Chloroform (CDC1 ) and diethyl ether were both obtained from Samchun, while paclitaxel (PTX) was purchased from Samyang Genex Corporation. A l l other reagents were of analytical grade and used without further purification.


2

3

Temperature-Sensitive Block Copolymer ( P C L A - P E G - P C L A ) and ( P C G A PEG-PCGA) The synthesis of the P C L A - P E G - P C L A and P C G A - P G E - P C G A block copolymers was performed through a ring-opening copolymerization reaction using P E G as an initiator and Sn(Oct) as a catalyst. The ratios of P E G / P C L A and C L / L A , P E G / P C G A and C L / G A were adjusted by altering the feed ratios of PEG, C L , L A and G A . The detailed synthesis was as follows: P E G and Sn(Oct) were added to a two-neck round-bottom flask and were dried for 4 h under vacuum at 110°C. After cooling the flask to room temperature, L A and C L or G A and C L were added under dry nitrogen, and the reactant mixture was dried for 1 h under vacuum at 60°C. Then, the temperature was raised slowly to 130°C, and the reaction was performed over a period of 24 h under dry nitrogen. The reactants were then cooled to room temperature, dissolved in methylene chloride (MC), and added to excess diethyl ether, causing the products to precipitate. The precipitated block copolymer was then dried under vacuum at 40°C over 48 h, affording a yield of over 75%. 2

2

Sulfamethazine Oligomer The sulfamethazine monomer (SMM) was synthesized from sulfamethazine (SM) and methacryloyl chloride. First, S M (0.1 mol) and sodium hydroxide (0.1 mol) were dissolved in aqueous acetone (100 mL, 1:1 v/v), and methacryloyl chloride (0.12 mol) was then added dropwise to the solution with stirring (0°C). The resulting mixture was then stirred for a further 3 h at 0°C. The precipitated



Ill S M M was filtered from the solution, washed with distilled water, and then dried under vacuum at room temperature for 48 h. The S M M yield was approximately 85% after drying. The sulfamethazine oligomer (OSM) containing a carboxyl acid end group was synthesized by conventional radical polymerization with S M M , A I B N , and 3-mercaptopropionic acid (MPA). The molecular weight of the O S M was controlled by altering the feed ratios of A I B N and M P A . The synthetic procedure was as follows: S M M (90 mmol) was dissolved in anhydrous N,Ndimethylformamide (DMF) (150 mL), after which A I B N (9 mmol) and M P A (9 mmol) were added under dry nitrogen to afford an S M M / A I B N / M P A mole ratio of 100/10/10. The temperature of the reactants was slowly increased to 85°C, and the reaction was carried out for 48 h. Subsequently, after evaporating the solvent (DMF), the resultant was redissolved in tetrahydrofuran (THF). The slow addition of the T H F solution to excess diethyl ether resulted in the precipitation of O S M , which was filtered and dried slowly at 40°C for 48 h. The yield (ratio of the weight of the final product (OSM) to the total feed amount of S M M and M P A ) was 90%.

Coupling of Sulfamethazine Oligomer with Temperature-Sensitive Block Copolymers The temperature-sensitive triblock copolymer and O S M were coupled together using 1, 3-dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino) pyridine (DMAP) as a catalyst. The coupling reaction process was as follows: The tri block copolymer (0.1 mol) was weighed into a two-neck flask and dried under vacuum at 85°C for 2 h. O S M (0.24 mol) was then added to the flask under dry nitrogen, and the reactant mixture was dried under vacuum at 85°C for 1 h in order to completely remove any moisture. The reactant mixture was cooled to room temperature under dry nitrogen and, then, an anhydrous M C solution (60 mL) containing D C C and D M A P was added to the flask using a glass syringe to afford a triblock copolymer/OSM/DCC/DMAP feed ratio of 1/2.4/2.8/0.28 mol. The reaction was carried out at room temperature for 48 h. Although O S M is insoluble in M C , it reacts with the triblock copolymer due to the high solubility of the triblock copolymer in M C . Over the course of the coupling reaction, D C C was slowly converted into dicyclohexylurea (DCU). The residual D C C was also converted into D C U by the addition of two or three drops of water, and the combined D C U byproducts were precipitated and removed (0.4 μηι filter paper) along with the residual O S M . The final product was obtained by pouring the filtered reactant mixture into excess diethyl ether, and the resulting precipitate was dried under vacuum at 40°C for more than 48 h to give a final yield of over 60%.


112 Characterization The number-average molecular weight ( M ) and molecular distribution ( M W D ) of the as-synthesized block copolymers were determined by gel permeation chromatography (GPC) measurement on a Waters Model 410, equipped with 4 μηι-styragel columns from 500 to 10 Â in* series, at a flow rate of 1.0 ml/min (eluent: T H F , 36°C, P E G as standard). *H-NMR measurements were performed on a Varian Unity Inova 500 instrument (500 M H z ) to determine the molecular structures and compositions of P E G , C L , and G A (30, 31).


n

Phase Diagram Measurements The block copolymers were dissolved at a given concentration in a buffer solution (in a 4 mL vial) for 1 day at 0°C. The buffer solution was prepared using PBS tablets and NaOH (0.9 wt %). The pH of the block copolymer solution was adjusted to a specific pH by adding small amounts of 5 M HC1 solution at 2°C. Each solution was kept at 4°C for 30 min in a water bath. The vial was then slowly heated in a water bath in intervals of 2°C. The vial was held at each temperature for 10 min to allow it to equilibrate and then laid down horizontally for a further 1 min. The sol (flow)-gel (no flow) phase-transition temperature of the block copolymer solutions was determined using this method. The measurements were repeated three times, and each point represented an average with an accuracy of 2°C.

Sol-gel Transition by In Vitro Test The penta-block copolymers were dissolved to obtain a solution at a concentration of 20 wt%, at pH 8.0 and 10°C. Then these solutions were injected into a 30 mL vial containing 20 ml of PBS buffer (pH 7.4 and 8.0) at 37°C through a spiral glass. Following this, the vials were shaken to check the ol-gel state of the sample.

Degradation of Block Copolymer The degradability of the block copolymers was determined by following the changes in the molecular weight over time. The block copolymer solution was prepared at 0°C using a method similar to that used for the phase diagram experiment and was adjusted to pH 7.4. 0.5 g of the triblock and penta-block solutions at 20 wt % in water (pH 7.4) were placed in a 4 ml vial and incubated


113 at 37°C for 30 minutes. PBS buffer (3 ml) at pH 7.4 and 37°C was added to the solution. Samples were then taken at designated time intervals and freeze-dried. The change in the molecular weight was determined by GPC.


Cytotoxicity Determination The cytotoxicity was characterized as a decrease in the metabolic rate measured using the X T T (2,3-bis(2-methoxy-4-nitro-5-susfophenyl)-2Htetrazolium-5-carboxanilide) assay (31, 32). Cells were plated in 96-well plates at an initial density of 10,000 cells/well in 100 \xL of growth medium (90% Dulbecco's modified Eagle's medium, 10% fetal bovine serum, 4500 mg/L glucose L-glutamine, 110 mg/L sodium pyruvate and sodium bicarbonate). The cells were grown for 24h, after which the growth medium was removed and replaced with fresh, serum-free medium containing the polymer. The cells was incubated with the polymer for 4 h at 37°C, and the medium was replaced with complete growth medium for 24 h. X T T labpling mixture was prepared by mixing X T T labeling agent (50 μ ι ) and an electron coupling agent (1 \xL) and 50 μΐ, of the X T T labeling mixture was added to each well. The samples were incubated for 4 h at 37°C under 5 % C 0 and the absorbance was read between 492 nm and 690 nm. 2

Drug Loading and Releasing Experiment The drug paclitaxel (PTX) was loaded into the penta-block copolymer solutions (20 wt % in water at pH 8.0) at 0°C over a period of 1 day. The sample pH was adjusted to pH 7.4 with sodium hydroxide (5 M) and HC1 (5 M ) and the solution maintained at 0°C for 12 hours. Subsequently, 0.5 g of the mixture was placed in a 4 ml vial and incubated at 37°C for 30 minutes. Fresh serum (3 ml, 2.4 wt % Tween 80, 4 wt % Cremophor E L in PBS buffer at pH 7.4) at 37°C was added to the vial samples. At a given time, 1.5 ml of the serum was extracted from the vial sample and freeze-dried. The amount of paclitaxel in the samples was determined by H P L C (Column: C I 8 , 250 χ 4.0 mm, 5.0 μχη; mobile phase: A C N / H Q = 2/8; flowrate: 0.5 mL/min; detector: U V at 254 nm). 2

Results and Discussion Synthesis and Characterization The molecular structures of the synthesized S M M and O S M were confirmed by H N M R , as shown by the corresponding spectra and peak assignments in !


114 Figure 1. The aromatic (c, d) and amine (e) protons shown at 7.65 ppm (c), 6.55 ppm (d), and 5.98 ppm (e) in the sulfamethazine (SM) spectrum (A) were observed to shift to 7.95 ppm (c), 7.85 ppm (d), and 10.08 ppm (e) in the corresponding S M M spectrum (B), respectively. In confirming the formation of O S M , the methyl signal (1.98 ppm, f) in the S M M spectrum is shifted upfield to 1.10 ppm ( f ) in spectrum (C), while the corresponding ethylene signals (=CH2, g, h) show a significant decrease in intensity. The peak of the hydrogen proton in the sulfonamide group ( S 0 N H ) is shown at 11.6 ppm in the *H N M R spectra for all three samples. These H N M R results show conclusively that S M M was successfully synthesized. On the other hand, further evidence is required to confirm the molecular structure of O S M . The molecular weight of O S M was controlled by adjusting the feed ratio of the monomer, the initiator, the transfer agent, and the reaction time. Table I shows the molecular weight of O S M , as determined by GPC (relative to P E G standards). 2


l

Table I. Sulfamethazine Oligomers Feed Ratio (mol ratio)" 1/0.1/0.1 (OSM,) 1/0.1/0.2 (OSM ) 1/0.1/0.1 (OSM ) 1/0.1/0.2 (OSM4) 2

3

a

Reaction Time (h) 48 48 40 40

b

M„ 1144 937 904 806

b

MJM„ 1.35 1.24 1.32 1.26

b

[monomer]/[initiator]/[transfer agent]. Measured by GPC relative to PEG standards.

Various PCGA-PEG-PCGA block copolymers were obtained by ring opening polymerization. The number average molecular weight (Mn) of the block copolymers can be calculated by comparison of the peak ratios of C L and G A with those of PEG (of known molecular weight) in the *H-NMR spectrum. Figure 2 shows a representative H - N M R spectrum of the PCGA-PEG-PCGA block copolymer and its chemical structure. The characteristic signal appearing at 3.6 ppm was assigned to the methylene protons of the EO units and those at the ends of the C L units, while the signals at 4.68 and 2.35 ppm correspond specifically to the methylene protons of the G A unit and those at the beginning of the C L unit, respectively. The molecular compositions of the synthesized block copolymers were obtained by calculating the corresponding peak areas (33). Various P C L A - P E G - P C L A block copolymers were obtained from the ringopening polymerization reaction. The number average molecular weight (Mn) of the block copolymers was calculated using the H N M R spectrum of a P E G standard of known molecular weight. Figure 3 shows the representative 1H N M R spectrum of the P C L A - P E G - P C L A block copolymer and its chemical structure. A l l of the proton signals of the block copolymer were assigned as !

!



Figure!.

5

Ή NMR spectra of sulfamethazine (A), sulfamethazine monomer (B), and sulfamethazine oligomer (C) in DMSO-d .



PPm

X_X_'

I

60

«

a

1

a

a

a* b

d

e

f

g a" h

I

I I

50

I,

t I

I

4.0

M

c h a b »

«

'

'

lai

>, t....»

3.0

a. a

L

'

«

ι

20

»„, .1,,«,

Figure 2. Ή-NMR spectrum of PCGA-PEG-PCGA. (Reproduced with permission from reference 31. Copyright 1993.)

• », „,l

l

I

I

1.0

• >

Jy

c V - o + c - c o k - c - o - o c o - 6 - c -c -c -c -c — o h

b



117

J

Figure 3. HNMR spectrum of PCLA-PEG-PCLA block copolymer in CDCl and its chemical structure.

3

labeled in Figure 3. Among the proton peaks, the methylene proton of the oxyethylene unit (A, A ' ) , the methine proton of the L A unit (D), and the methylene proton (on the neighboring carbonyl group) of the C L unit (E) were used to calculate the Mn and composition of the block copolymer according to the method described in reference (34). O S M was coupled with the temperaturesensitive block copolymers ( P C L A - P E G - P C L A ) or (PCGA-PEG-PCGA) using D C C and DMÂP. The synthesis of the O S M - P C L A - P E G - P C L A - O S M or O S M - P C G A - P E G P C G A - O S M block copolymer was confirmed using *H N M R and GPC. The H N M R spectrum of O S M - P C L A - P E G - P C L A - O S M and O S M - P C G A - P E G P C G A - O S M show aromatic protons (7.6-8.0 ppm, c,d) and an imidazole ring proton (around 6.8 ppm, b), which are the typical signals associated with O S M (Figure 4). Also, the molecular weights of O S M - P C L A - P E G - P C L A - O S M and O S M - P C G A - P E G - P C G A - O S M showed a significant increase compared to those of P C L A - P E G - P C L A and P C G A - P E G - P C G A (Figure 2, 3). Tables II and III show the molecular weights of the triblock and penta-block copolymers. In addition, the G P C traces of O S M , P C L A - P E G - P C L A and O S M - P C L A - P E G P C L A - O S M copolymer show a narrow molecular weight distribution (Figure 5). These results demonstrate that a block copolymer with a narrow molecular weight distribution was successfully synthesized. l



118



119

a:

Ik


120


Table Π. Physieal parameters of PCLA-PEG-PCLA and OSM-PCLA-PEGPCLA-OSM block copolymers OSM-PCLA-PEG-PCLA-OSM

^ M 1500 1500 1500 1750 1750 2000 2000

OSM, -1384-1500-1384-OSM, (a-1-1) OSM, -1554-1500-1554-OSM, (a-2-1) OSM -1554-1500-1554-OSM (a-2-2) OSM, -1642-1750-1642-OSM, (b-1-1) OSM, -1823-1750-1823-OSM, (b-2-1) OSM, -1856-2000- 1856-OSM, (c-1-1) OSM,-2104-2000-2104-OSM, (c-1-1) 2

2

a

n

?B

f^ (w/w) 1/1.85 1/2.08 1/2.08 1/1.89 1/2.08 1/1.86 1/2.10

mol/mot 2.44/1 2.59/1 2.59/1 2.44/1 2.49/1 2.64/1 2.71/1

° ^ M„ 1144 1144 937 1144 1144 1144 1144

MM n

1.45 1.48 1.46 1.50 1.53 1.54 1.56

PCLA-PEG-PCLA number-average molecular weights were calculatedfrom'H-NMR.

Provided by Aldrich. "Measured by GPC.

Table III. Physical parameters of PCLA-PEG-PCLA and OSM-PCLAPEG-PCLA-OSM block copolymers OSM-PCGA-PEG-PCGA-OSM OSM -1091 -1000-1091 -OSM (A-1 ) OSM4-1590-1500-1590-OSM4 (B-l) OSM4-2474-2000-2474-OSM4 (D-l) OSM4-1881-1750-1881 -OSM (C-2.1) OSM3-1461 -1750-1461 -OSM (C-1 ) OSM3-1881-1750-1881 -OSM (C-2) 4

4

4

3

3

OSM3-2118-1750-2118-OSM3 (C-3)

OSM -2336-1750-1336-OSM (C-4) OSM3-2494-1750-2494-OSMj (C-5) 3

3

PEG Ma

1000 1500 2000 1750 1750 1750 1750 1750 1750

PEG/PCGA CL/GA OSM MJM; mol/mof Μ η (W/W)° 2.34/1 806 1/2.18 1.30 1/2.12 2.35/1 806 1.34 1/2.47 2.37/1 806 1.50 1/2.15 2.32/1 806 1.35 1/1.67 2.26/1 904 1.35 1/2.15 2.32/1 904 1.35 1/2.42 2.26/1 904 1.36 1/2.67 2.29/1 904 1.36 1/2.85 1.35 2.28/1 904 l

'PCGA-PEG-PCGA number-average molecular weights were calculatedfromH-NMR. Provided by Aldrich. Measured by GPC. SOURCE: Reproduced with permissionfromreference 31. Copyright 1993. c



121

Retention Time Figure 5. GPC traces. (A) PEG (Mn ) 1500), (B) PCLA-PEG-PCLA (C) OSM-PCLA-PEG-PCLA-OSM (a-1-1).

(A-l),

Sol-Gel Phase Transition The sol-gel transition phase diagrams of the block copolymer solutions were investigated under various p H and temperature conditions. Figure 6 shows the sol-gel transition mechanism of the penta-block copolymers. The copolymer solutions pass through 4 stages depending on the temperature and p H conditions. A t low temperature (10°C) and high p H (8.0), P C L A or P C G A are not sufficiently hydrophobic and O S M is ionized and easy to dissolve. Therefore, the P C L A - O S M or P C G A - O S M block copolymers are slightly hydrophobic. For this reason, very small amounts of micelles form (stage D) and the copolymer solutions stay in the sol stage. When the temperature increases to 37°C (stage B), P C L A or P C G A become more hydrophobic, but O S M remains in the form of a hydrophilic block; i f the p H decreases to 7.4 (stage C), O S M is de-ionized and becomes hydrophobic. Consequently, the hydrophobicity of P C L A - O S M or P C G A - O S M is increased and more micelles are formed, however the hydrophobicity of these blocks is not strong enough to form a hydrophobic link which would act as a bridge between the micelles, so the copolymer solutions still stay in the sol phase, although their viscosity increases. When the temperature is increased to 37°C and the p H reduced to 7.4 (stage A ) both O S M and P C L A or P C G A become more hydrophobic, and the hydrophobicity of P C L A - O S M or P C G A - O S M is strong enough to form a lot of bridges between the micelles, with the result that the copolymer solutions stay in the gel phase.



122

Figure 6. Sol-gel mechanism of the novel pWtemperature sensitive hydrogel. a) Sol-gel transition phase diagram, b) The mechanism of sol-gel transition. Continued on next page.



123

Figure 6. Continued.


124


Effect of Molecular Weight of OSM Figure 7 shows the sol-gel diagrams of the O S M - P C L A - P E G - P C L A - O S M and O S M - P C G A - P E G - P C G A - O S M block copolymer solutions as a function of the molecular weight of the O S M component. In this figure, the critical gel pH (CGpH) and the lower (sol to gel transition) and upper (gel to sol transition) critical gel temperature (CGT) curves are shown. It is found that O S M , which is present mainly in an ionized state in the high-pH range, is present in the sol state regardless of its molecular weight. However, it can be seen that a decrease in pH results in the formation of a non-ionized O S M , which thus acts as a hydrophobic block. Also, particularly in the low-pH range, the hydrophobicity of the block copolymer increases with increasing molecular weight of O S M , resulting in an overall increase in the temperature range at which the gel forms. As a result, the C G p H in the phase diagram of C-2 is higher than that of C-2.1 (Figure 7a), and the C G p H in the phase diagram of a-2-1 is higher than that of a-2-2 (Figure 7b). In the case of almost every sample, when the temperature is increased from below the lower C G T to above it, the solution of the pH/temperature sensitive block copolymer transforms from the sol to the gel phase in a single stage. However, when the temperature is increased from below the upper C G T to above it, the gel-sol transition and suspension phase occur concurrently. When the temperature is greater than the upper C G T , the enthalpy of H 0 at this temperature is too high, so that the water is effectively liberated from the gel matrix. 2

Effect of PEG Molecular Weight Figure 8 shows the changes in the sol-gel diagrams that occur as the molecular weights of PEG and the block copolymer increase, in the case where the molecular weight ratios of the hydrophilic (PEG) and hydrophobic (PCGA) or (PCLA) components are fixed. When the molecular weight of P E G was increased from 1000 to 2000, the lower C G T at a concentration of 10 wt% and pH 7.4 increased from 10°C ( A - l ) to 39°C (D-l), and the upper C G T increased from 38°C to 54°C (Figure 8a). Also, the lower C G T at a concentration of 15 wt% and pH 7.4 increased from 19°C (a-2-1) to 38°C (c-2-1), and the upper C G T increased from 45°C to 58°C (Figure 8b). It was found that the sol-gel phase diagram of the block copolymer moved toward higher temperatures with increasing block copolymer molecular weight at the same P E G / P C L A and P E G / P C G A ratio, yet revealed little or no change in the temperature range at which the block copolymer formed a gel. This suggests that when the length of the block copolymer is increased with a constant ratio of hydrophobic to hydrophilic blocks, gel formation by the block copolymer becomes possible, due to the more strongly hydrophobic conditions (i.e., strongly hydrophobic



125

7Μ

72

ΊΑ

7.6 ρΜ

7.8

&0

8.2

Figure 7. a) Sol-gel phase diagrams ofOSM-PCGA-PEG-PCGA-OSM (C-2, C-2. 1) block copolymer solutions with different molecular weight ofsulfamethazine oligomers. Mn ofPEG =1750; PEG/PCGA= 1/2.18 (w/w); concentration) 10%. b) Sol-gel phase diagrams ofOSM-PCLA-PEG-PCLA-OSM (a-2-1, a-2-2) block copolymer solutions with different molecular weight of sulfamethazine oligomers. Mn of PEG =1500; PEG/PCLA= 1/2.08 (w/w); concentration ) 15%..


126


a)

7.6 H (C:10wt%) P

b) PEGs1M0(»M)

70

• •

PEG*1?50(b.2-1> PEG*2000

pH and Temperature Sensitive Block Copolymer Hydrogels Based on

Recommend Documents