Structure and Properties of AB Cross-Linked Polymers - American

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28 Structure and Properties of AB Cross-Linked Polymers Wenzhong Liu, Xiaozu Han*, Jingjiang Liu, and Huarong Zhou Changchun Institute of Applied Chemistry, Academia Sinica, Changchun, People's Republic of China 130022

Hydroxy-terminated oligomers, such as polyester, polyether, or castor oil, were reacted with toluene diisocyanate to form isocyanate -terminated prepolymers, which were then reacted with 2-hydroxyethyl acrylate to obtain vinyl-terminated prepolymer (VTP). A series of AB cross-linked polymers (ABCP) was synthesized by free radical copoly merization of these VTPs and vinyl monomers using photo- or redox initiators. The effects of the cross-link density, component ratio, net work structure and defect, hydrogen bond, and monomer structure on the glass-transition temperature, relaxation, and mechanical properties of ABCPs were studied. The morphologies and dynamic mechanical properties of an ABCP and an interpenetrating polymer network (IPN) with the same composition were compared. The results showed that the degree of phase separation of the ABCP is lower than that of the IPN.

THE PHYSICAL PROPERTIES AND THE PROCESSABILITY OF POLYMERIC materials

can be improved by physical or chemical combination of two or more structurally different polymers. The simplest combination method involves mechanical blending of two polymers in an extruder or roll mill. These blending techniques generally lead to polymer Α-polymer Β systems that are not bonded to each other. In the important cases, soluble and thermoplastic polymer materials are prepared from more than one type of monomer unit in

* Corresponding author.

0065-2393/94/0239-0571$07.00/0 © 1994 American Chemical Society

In Interpenetrating Polymer Networks; Klempner, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

572

N I TERPENETRATING POLYMER NETWORKS

which monomer units are attached in long sequences. These materials include certain types of copolymers and their blends with one or more homopolymers. Examples of the simplest and most relevant copolymer species in this category that are based on two monomers, A and B, are hnear A B and A B A block copolymers and nonlinear block copolymer of A B . There are two different ways to prepare thermoset combinations of polymers. One case, where two different networks, A and B, are synthesized, is called an interpen etrating polymer network (IPN). In the second case, two polymers are grafted together to form one network. Polymer Β is bonded to polymer A at both ends or at various points along the chains. This network is called an A B cross-linked polymer (ABCP). Thus, ideally, one network is generated for A B C P compared to two different networks ideally synthesized for I P N . In ABCPs, although polymer A is bonded primarily to polymer B, it is not cross-linked to itself. Ideal IPNs are unique blends of two cross-linked polymers that contain essentially no covalent bonds or grafts between net works (1-3). To distinguish the type of bonding in A B C P s from the case where the product remains thermoplastic, we designated it conterminous hnking. Bamford and Eastmond (4-8) described a general preparation method for ABCPs of known structure: react a preformed polymer (the A compo nent) that has reactive halogen in side chains with a metal carbonyl in the presence of a monomer. The metal carbonyl specifically removes halogen atoms from the preformed polymer to produce radical sites on the side groups of the A chains. These macroradicals initiate polymerization of the monomer that produces growing chains of polymer Β attached to the A chains. Combination termination of the propagating grafts produces Β cross links, whereas disproportionation termination generates Β branches on A chains. In another case of A B C P preparation, a precursor polymer A that has reactive functions, such as organometallic sites, alcohol functions, and vinyl or hydrosilane groups, at both ends is coupled with a multifunctional polymer B. A typical example of the chain-joined A B C P (random A B C P ) is the graft copolymer of unsaturated ester and styrene monomer. This copolymer has been available commercially as a composite material binder for a long time. The regular A B C P is formed by the copolymerization of vinyl-terminated oligomer and vinyl monomer (9). In other pubhcations (JO, 11), we have described the syntheses, morphology, and dynamic mechanical properties of A B C P s based on polyurethanes. Although A B C P s have been investigated and reviewed during the past 20 years (5, 12), the details of the relationship between structure and properties of ABCPs are still incomplete. We have synthesized and studied a series of A B C P s with different structures. The effects of cross-link density, component ratio, network struc ture and defect, main chain structure, side chain structure, and hydrogen bonding on the properties of some ABCPs are presented in this chapter.


2


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AB Cross-Linked Polymers

Experimental Details


Materials. The names and properties of the materials, as well as the acronyms used in the text to designate the materials, are listed in Table I. Some of the materials were purified before utilization. Syntheses. Isocyanate-Terminated Prepolymer (ITP). A hydroxyterminated polymer (PEA, PPG, or CO) was heated to 100 °C while stirring in a triple-neck flask and degassed under vacuum for 1 h. After cooling to 60 °C under N atmosphere for 1 h, the polymer was reacted with a calculated amount of T D I ( N C O - O H = 2): 2

NCO H0-R-0H

* 2 0 C N ^ Q K H

ο

OCN 3

— ^ H

3

0

NCO

C - Q ^ N H C 0 - R - 0 C N H ^ Q > - C H 3

(ITP) Vinyl-Terminated Prepolymer (γγρ). The previously synthesized ITP was reacted with a calculated amount of H E A ( O H - N C O = 1.2) using 0.1 D B T D L as a catalyst at 60 °C for 1.5 h, to yield VTP: 0 ITP

• 2CH2=CHC0CH2CH20H—-

0

0

0

CH2=CHC0CH2CH20CNH H

3

Q

g

0

NHC0CH2CH20(!cH=CH2

C-^^NHC0-R-0CNH^Q-CH

(

2

)

3

(VTP)

ABCP. VTP, M , and a proper amount of B E E were weighed into a beaker, mixed thoroughly, degassed under vacuum, and then poured into a 3- X 10-mm mold and irradiated with 400-W U V lamp for 10 min. Then the obtained samples were heated at 100 °C for 1 h:

r

Î

til

Μ

M M V T P + nM ^ C H o - C H WVWNAAAAW C H — C H ?

ΐ

(ABCP)

(3)

if

IPN. A P E A P U - P M M A IPN was synthesized according to the following procedure: M M A , D V B , and C H P were weighed into a vessel and mixed. Then suitable amounts of PEA, T H M P , and D B T D L were added and dissolved. T D I


574

N I TERPENETRATING POLYMER NETWORKS Table I. Materials Name and Properties

Acronym PEA

Hydroxy-terminated polyethylene glycol adipate; molecular weight 2083 or 1345; O H value = 9.6 X 10~ or 14.87 Χ 10" mol/g Hydroxy-terminated polypropylene glycol; molecular weight 1295; O H value = 14.39 X 10~ mol/g Castor oil 2,4-Toluene diisocyanate Phenyl isocyanate 2-Hydroxyethylene acrylate Dibutyltin dilaurate Divinyl benzene 1,1,1 -Tris-(hydroxymethyl)-propane Benzoin ethyl ether Cyclohexanone peroxide Cobalt(II) naphthenate Vinyl monomer Methyl methacrylate Poly(methyl methacrylate) Butyl methacrylate Poly(butyl methacrylate) Acrylic acid Methyl acrylate Butyl acrylate Styrene Vinyl acetate Acrylonitrile Methyl ethyl ketone Polymerized vinyl-terminated prepolymer of PEA Polymerized vinyl-terminated prepolymer of PPG Polymerized vinyl-terminated prepolymer of castor oil 4

4

PPG


4

CO TDI PI HEA DBTDL DVB THMP BEE CHP CoN M MMA PMMA BMA PBMA AA MA BA ST VAc AN MEK PEAPU PPGPU COPU

and CoN were then added with stirring. The mixture was degassed, poured into a mold, and cured at room temperature for 24 h and at 80 °C for another 2 h. Solvent Extraction. In some cases, A B C P samples were extracted with benzene in a Soxhlet extractor for 24 h. The extracted solution was distilled under vacuum and the residue was analyzed by IR spectroscopy. Measurements. Dynamic Mechanical Properties. Dynamic mechanical properties (e.g., storage modulus E , loss modulus E", and tan δ) were measured using a viscoelastometer (Rheovibron model DDV-II-EA) with a frequency of 3.5 Hz and a heating rate of 3 °C/min. The test specimens were cast and cut into 25- X 4- X 0.3-mm slabs. The temperature that corresponds to the peak of tan δ was considered as the glass-transition temperature determined by dynamic me chanical method, T . When there were two T s, the T at higher temperature was defined as T and the T at lower temperature was defined as T . f

gd

gdl

gd

gd

gd


gd2

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MorphoL·gy. The morphology was studied on a transition electron micro scope (Hitachi H-500). The sample was cut under refrigeration and stained with Os0 . 4

Mechanical Properties. Stress-strain measurements were carried out on a tensile tester (Instron 1121) at 20 °C with a crosshead speed of 500 mm/min.


Results and Discussion Cross-Link Density. The cross-linking reactions in the syntheses of P E A P U - P M M A A B C P s take place at the ends of V T P . This hnking is different from the more common unsaturated polyester A B C P s . If the preceding reactions 1-3 were all completed and reaction 3 were terminated by combination, a perfect A B C P network would be formed as shown schematically in Figure 1. One P E A P U segment and two P M M A segments can form a repeat unit. The molecular weight of the P E A P U segment, which is equal to that of V T P ( M ) , can be calculated from the molecular weight of the P E A used. The ratio of weight fraction of the two components of an A B C P is expressed as r. When the r value is known, the cross-link density of a n A B C P that is exhibited by the average molecular weight between cross-links ( M ) can be calculated from the equation W P

c

M

c

= (l + r)M

V X P

/3

(4)

M was also determined by both swelling and plateau modulus methods. A B C P based on P E A P U and P M M A was swelled in M E K . The relationship c

Figure 1. Schematic diagram of ABCP network.


576

INTERPENETRATING POLYMER NETWORKS

between the equilibrium swelling and the cross-link density can be expressed by the Flory-Rehner equation (13): - l n ( l - Φ0 - φ - Χφ\ = pV /M (^ χ

s

(5)

- 2φ,//)

3

c

where ρ is the density of A B C P after extration, V is the molar volume of solvent, φ is the volume fraction of the gel in the swollen A B C P , X is the polymer-solvent interaction parameter, and / is the cross-linking functional ity. The X value in eq 5 can be calculated (13) from s


λ

X = β + V /RT(b 1

s

s

- δ ) ρ

(6)

2

where β is the lattice constant (usually 0.35 + 0.1), R is the gas constant, Τ is the absolute temperature, δ is the solubility parameter of the solvent with a value of 19.0 J / m L for M E K (14), and δ is the solubility parameter of the A B C P . The measured δ value of P E A P U was 21.08 J / m L and the reported δ value of P M M A is 19.44 J / m L (14). The δ value of the P E A P U - P M M A A B C P , which is an approximately compatible system, can be obtained according to the simple addition principle: χ

δ

1 / 2

1 / 2

ρ

1 / 2

1 / 2

δ = δρ ρ

Ε Α Ρ υ

φρ

+ δ

Ε Α Ρ υ

1 / 2

1 / 2

Ρ Μ Μ Α

φ

(7)

Ρ Μ Μ Α

where φ is the volume fraction. The M of the P E A P U - P M M A A B C P s with different component ratios, determined from eq 5 and shown in Figure 2, are lower than the values calculated from eq 4. The shear plateau modulus (G) of an elastomer has the following relationship with cross-link density (N) and absolute temperature (15): c

G = JVRT= ( p / M ) R r

(8)

c

Because Young's modulus (E ) is 3 G , f

£' = 3ρΚΓ/Μ

(9)

0

where ρ is the density of the elastomer, which can be calculated from the volume expansion coefficient, β, according to ρ = p0/(l + β(Γ - T0))

(10)

where T = 293 Κ and p is the density of the sample at T . Figure 3 shows the E' — Τ dependence for P E A P U - P M M A ABCPs with differing composi tions. Samples I-IV all have rubber plateaus, which indicates the existence of network structure. The plateau moduli decrease with increasing M M A con tent, which reduces the cross-link density. The M values were calculated using eq 9 from the plateau Young's moduli at the plateaus; the temperature 0

0

0

c



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0.2

0.6 MMA,Wt% 0.4

Figure 2. Relationship between M and MMA content in PEAPU-PMMA ABCPs. —, calculated from eq 4; ·, measured by the swelling method; O, measured by the plateau method. c

at the inflection point plus 10 Κ was taken as the reference temperature. These M values are also lower than the values calculated from eq 4, but coincide with the values measured by the swelling method (see Figure 2). For the calculation of M , it was assumed that all of the V T P and M M A entered into the network; in reality, this is impossible. In the experiments about 3 % M M A monomer did not polymerize and 9% M M A formed soluble polymer (sol). When the A B C P samples were swollen in solvent, unreacted monomer and sol were extracted. Thus the M values measured by both the swelling method and the plateau modulus method are values of effective molecular chains. These values agree well with each other, but are lower than the calculated values. c

c

c

Effects of Cross-Link Density and Component Ratio on D y namic Mechanical Properties. The most convenient method to study the effect of the cross-link density of the P E A P U - P M M A A B C P s on the dynamic mechanical properties is to change the molecular weight of P E A . The results are shown in Figure 4. The damping peaks are at higher temperatures of 98 and 115 °C for A B C P s with P E A molecular weight of 2083 and 1345, respectively. The Τ in the lower temperature range, which d 2


578


3 A

D


χ Ο Δ Δ · V,Ν "/ Δ · Ο Δ · Δ · * Ο Δm ·

Α

3

y. Ο

Α Ο Α Ο Α

•Α ·

,Η

°Α

O O

> CvKL

· Φ

u

O m ι

D

oo

1

O

H I

3

d



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583


MMA.Wt% Figure 7. Relationship between Τ and the composition of ABCPs. d l

PEAPU-PMMA

ing agent, the P E A used must have a molecular weight of 1458. Use of the proper amount of D V B as the cross-linking agent yields M = 1895 for the P M M A network. The dynamic mechanical properties of A B C P s and IPNs with the same compositions and the same cross-link densities are shown in Figure 8. The glass-transition and relaxation behavior of A B C P s and IPNs are quite different. The I P N has two obvious T s , which appear at temperatures that correspond to the T s of P E A P U and P M M A , and a plateau between them. This configuration indicates that phase separation occurred between the two networks. However, the P E A P U - P M M A A B C P shows one T (at 95 °C) with a shoulder, which indicates a higher degree of miscibility (more homogeneous) than I P N . The dynamic mechanical spectra in Figure 8 reveal that the Young's moduli of the IPNs are smaller than the moduli of A B C P over a wide temperature range ( —100 to + 70 °C), especially around T . Such a great difference between the Young's moduli of A B C P and I P N reveals that P E A P U is the continuous phase in the I P N , but P M M A is continuous in ABCP. A secondary relaxation between —100 and —80 °C in A B C P , as well as in the I P N , that is due to the local movements of P E A P U segments that are unrelated to the network structure can be seen in Figure 8. c

d

g

gd

gdl


584

N I TERPENETRATING POLYMER NETWORKS beoooQooooo ο ο Q Ο Ο

••ABCP

Q


Q

ΙΡΝ\

'V. #

PEAPU

Δ Δ PMMA Δ Δ Δ IPN \ Δ

ABCP

Δ

"θ

•

Ο»

-100

-50

β

·

Ο

Δ

50

Δ

Δ

Δ

100

150

200

T,°C Figure 8. Comparison of dynamic mechanical spectra between ABCP and IPN of PEAPU-PMMA. MM A: VTP = 50:50; calculated M = 1895. c

Morphologies of ABCP and IPN. Transmission electron micro graphs ( T E M ) of ABCPs and IPNs with the same composition show a two-phase nature (Figure 9). In the I P N , the P E A P U phase is continuous (dark areas) and the P M M A phase is dispersed (light areas), which is in agreement on the foregoing results. The dispersed phase appears to be spherical structures with quite different domain sizes ranging from 0.03 to 0.55 urn; most spheres are from 0.08 to 0.15 μπι. The domains do not have clear boundaries, especially the larger domains, which reveals that interpéné tration may occur at the boundary. A P E A P U fine structure was found in the



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matrix and across the smaller dispersed domains, which implies that interpén étration occurred not only at the boundaries, but also inside the domains. In P E A P U - P M M A A B C P , P M M A is the continuous phase and P E A P U is the dispersed phase with irregular shapes (Figure 9B). During the forma tion of the A B C P network, the aggregation of P E A P U segments is impeded by the chemically bonded P M M A segments because the P E A P U is joined at the tail to P M M A . Phase separation occurs over a limited range, which results in the formation of smaller domains with sizes less than 0.01 urn. These morphologies are completely in agreement with the results of the dynamic mechanical studies. Effects of Sol i n P P G P U - P S T A B C P . A viscous material was obtained from the extract phase of P P G P U - P S T A B C P extraction. IR analysis showed that the sol content primarily came from P P G . The function ality of P P G used here was 1.9, so some P P G molecules, especially those with monohydroxyl groups or with no hydroxyl groups, cannot enter the cross-lin ked network. The dynamic mechanical properties of A B C P samples before and after sol extraction are illustrated in Figure 10A and B, respectively. The original sample had. a primary glass transition at 55 °C and another at — 70 °C. The existence of two T s is evidence for phase separation. If P P G P U and PST were mixed at a molecular level in each phase, then the components of each phase could be estimated from the Fox equation (16): gd

1/T = W./T^ + W,/T g

(12)

g2

where T , T , and T represent the glass-transition temperatures of a certain phase of A B C P and two homopolymers, respectively, and W and W represent the weight fractions of the two components in this phase, respec tively. The calculated results are listed in Table III. The T at the higher temperature corresponds to the dynamic glass-transition temperature of the PPGPU-plasticized polystyrene (PST) phase, whereas the lower T corre sponds to the PST-reinforced P P G P U phase. The sample after sol extraction (Figure 10B) has a primary glass transition at 96 °C (41 °C higher than before extraction) and a shoulder at 75 °C, as well as another glass transition at —12 °C (5 °C lower than before extraction). Clearly, sol extraction enhances the phase separation of P P G P U - P M M A A B C P . g

gl

g2

2

x

gd

gd

Effects of F r e e E n d on the Properties of C O P U - P B M A A B C P . When A B C P is prepared, some free ends in the network are formed due to such complex factors as lower functionality of the oligomer diol, chain transfer, and termination by disproportionation in reaction 3. To control the free-end content easily, castor oil (CO) was used as the polyhy-




586

Figure 9. Transition electron micrographs of ABCP and IPN of A,IPN;B, ABCP.

PEAPU-PMMA.



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Figure 10. Effect of sol extraction on tan δ-Ύ spectra of PPGPU-PMMA ABCP. PPGPU:PMMA = 50:50; A, original; B, after sol extraction. Table III. Phase Components of P P G P U - P M M A A B C P Spectrum Β

Spectrum A Phase (wt%)

PPGPU at -70 °C

PST at 55 °C

PPGPU at -12 °C

PST at 96 °C

PPGPU PST

72.6 27.4

24.8 75.2

77.4 22.6

2.0 96.0

droxyl compound to obtain C O P U - P B M A A B C P . Castor oil has three hydroxyl groups, and if it reacts with the proper amount of phenyl isocyanate (PI), chains capped with PI cannot enter the network and have a free end. Then the amount of PI added will be the concentration of free ends in ABCP. Five samples with free-end concentrations of 0, 1.5, 3.2, 5.0, and 7.2 X 10 ~ mol/mL were obtained by changing the PI amounts. The effects of the free-end concentration on the stress-strain behavior of A B C P s are illustrated in Figure 11. The rubber elasticity theory states that only the effective chains cross-linked in the network respond to the stress when the network is stretched, whereas the free-end chains do not respond. Therefore, the Young's moduli are decreased and the elongations are increased with increasing concentrations of free ends in the network. Figure 11 also shows that yield points in the stress-strain curves disappear gradually with increas4



588

N I TERPENETRATING POLYMER NETWORKS

curve 1, 0; curve 2,1.5; curve 3, 3.2; curve 4, 5.0; curve 5, 7.2. ing concentration of free ends; the A B C P with a concentration of 7.2 X 1 0 " mol/mL become a typical elastomer. The dynamic mechanical properties of the four samples with given concentrations of free ends are shown in Figure 12. A l l of the C O P U - P B M A A B C P s have one T that shifts to the lower temperature region with increasing free-end concentration. The thermodynamic theory implies that C O P U - P B M A A B C P s with different concentrations of free ends at their glass-transition temperatures have approximately equal free volume fractions, 0.025 (17). The free-ended chains in C O P U - P B M A ABCPs have larger free volume than the effective chains in the network at the same temperature. The free volume increases with increasing temperature. Therefore, the A B C P s with higher concentrations of free ends can get the same free volume at a lower temperature, which means they have lower T s. In Figure 12 it is also shown that tan δ values at any temperature below the T s increase with increasing concentrations of free ends and the damping factors (tan δ maximum) increase from 1.02 to 1.42 because the E' value is decreased from 11.0 X 1 0 " to 4.6 X 10 ~ Pa with increasing concentrations from 1.5 X 1 0 " to 7.2 X Μ Γ mol/mL. 4

gd

gd

gd

6

4

6

4

Effects of Structures of H a r d Segments o n the Properties of A B C P s . Vinyl monomers form the hard segments of ABCPs after copo lymerization with VTPs. The structure of the vinyl monomers effects the properties of the ABCPs. In the τ^ιηδ-Τ spectra of A B C P s that contain



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T,°C Figure 12. Effect of free ends on the dynamic mechanical properties of COPU-PBMA ABCPs. Curves 2-5 have the same concentration values as in Figure 11. P E A P U and acrylic esters (curves 1 and 2 in Figure 13), one T was found in each curve. However, the microscopic phase separation observed from the T E M results shows the immiscibility at the molecular level of the two components in PEAPU-poly(acrylic ester) ABCPs. The T of P E A P U - P B A A B C P is lower than that of P E A P U - P M A A B C P because P B A has longer and more flexible side groups. When methacrylates were used as a component i n the ABCPs, for example, P E A P U - P M M A A B C P , a tan δ - Τ curve quite different from the curve of P E A P U - P M A A B C P (see curves 3 and 1 i n Figure 13) results because the presence of a methyl group in the monomer. P E A P U - P M M A A B C P has a higher T and a wider damping peak range than P E A P U - P M A ABCP. M A and V A c are isomers. In ABCPs of P P G P U - P M A and P P G P U PVAc, P M A and PVAc show obvious differences in miscibility with P P G P U gd

gd

gd



0

Λ

T.°C

Δ

π

50

\ * Μ. Δ

χ

ΔΔΔ

>

φ

3 Λ

100

Α

V

150

Figure 13. tan δ—Τ spectra of ABCPs. VTP-.vinyl monomer = 50:50. Composition: curve 1, PEAPU-PMA; curve 2, PEAPU-PBA; curve 3, PEAPU-PMMA; curve 4, PPGPU-PMA; curve 5, PPGPU-PVAc; curve 6, PEAPU-PAN.

•50

*

ο

.Λί*··

ί^°0

• Δ

•

•Δ Δ

π°

Δ»

® Λ

σ

Θ ©

ο

ο

ο ·

o°°o.


28.

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(see curves 4 and 5 in Figure 13). A single glass transition at 28 °C is found in curve 4, whereas two T s at 21 and 42 °C are found in curve 5 and a plateau appears in a 64 °C temperature span. The tan δ maximum of P P G P U - P M A is much higher than the tan δ maximum of P P G P U - P V A c . A l l the facts demonstrate that the sequence of the side-chain atoms can affect the mechanical damping and the miscibility seriously. Curve 6 in Figure 13 demonstrates that A B C P obtained from P E A and A N have two independent T s: T is — 21 °C, which corresponds to the T of P E A P U ( - 2 7 °C), and T is 98 °C, which corresponds to the T of P A N (104 °C) (18). The existence of the two definite T s shows the P E A P U - P A N A B C P is an immiscible system. gd


gd

gd2

g

gdl

g

gd

Effect of Hydrogen Bonding in P E A P U - P M M A + AA ABCP. The chemical components and the main side-chain structures are the major factors that influence the properties of polymer materials, but the effects from secondary valence force may not be ignored i n the polar polymers. The hydrogen bond is the most important representative of secondary valence force. The urethane groups in polyurethane have high cohesive energy (36.6 KJ/mol) (19) and form hydrogen bonds with each other. This important factor effects the mechanical properties of the materials (20). To study the effects of hydrogen bonding in A B C P , another vinyl monomer, acrylic acid, that has a stronger ability to form hydrogen bonds was added to the P E A P U - P M M A system. The compositions of these ABCPs are listed in Table IV and the corresponding tan b-T spectra are shown in Figure 14. The parameters obtained from data shown in the figure are also listed in Table IV. No obvious change of the main glass-transition temperature ( T ) is seen in Table IV and Figure 14 when the A A content is increased gradually to 10%. This stability is because the T s of P A A (106 °C) and P M M A (105 °C) are approximately equal (18). The addition of A A to P E A P U - P M M A A B C P changes the miscibility significandy. A n obvious shoulder is found at 45 °C beside the main T in the tan b-T curve of P E A P U - P M M A A B C P without A A (curve 1 in Figure 14). The shoulder is the result of partial superimposition of the damping curves l a and l b . (The dashed lines of curves l a and l b are drawn according to the symmetry principle.) The shoulder indicates that P E A P U - P M M A A B C P is a two-phase system. The miscibilities of P E A P U - P M M A + A A ABCPs with gdl

g

gd

Table IV. Compositions and Some Parameters of P E A P U - P M M A + AA ABCPs No. 1 2 3

VTP:(MMA + AA) 50:(50 + 0) 50:(45 + 5) 50:(40 + 10)

V

(°c) 72 72 73

T>

(°c) 45

— —

tan δ maximum 0.79 0.92 0.98



Figure 14. Effects of AA content on the dynamic mechanical properties of PEAPU-PMMA + AA ABCPs. VTP:(MMA + AA) = 50:50. AA content (%): curve 1, 0; curve 2, 5; curve 3, 10.



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less than 10% A A are increased significantly and the shoulder at 45 °C disappears. The main reason for this change is the hydrogen bonds formed between soft and had segments, — C O O H ··· 0 = C N H — , by the addition of A A monomer. Another reason may be that the solubility parameter of the hard segment is enhanced by the addition of polar monomer A A and is closer to that of the P E A P U soft segment. Hydrogen bonds can absorb mechanical energy and can be broken by external forces; they can also regenerate with release heat energy. Therefore, the damping factor, tan δ maximum slightly increases with the introduction of AA, which forms stronger hydrogen bonds in A B C P .

Summary A series of ABCPs with differing compositions and cross-fink densities was synthesized by copolymerization of vinyl-terminated oligomers and vinyl monomers. The cross-link density was found to be one of the most important factors that effects the dynamic mechanical properties and the miscibility. The increase of cross-link density enhanced the T values and decreased the tan δ maxima of P E A P U - P M M A ABCPs. The T can be varied in a wide temperature range by alteration of the component ratio. A comparison of the dynamic mechanical properties and the morpholo gies of the P E A P U - P M M A ABCPs and IPNs with the same composition and cross-link density revealed that the degree of phase separation of ABCPs is lower than the degree of phase separation of IPNs. The sol fraction i n P P G P U - P S T ABCPs improved the interphase misci bility and enhanced the tan δ maximum. The free ends in C O P U - P B M A A B C P networks also increased the damping factor. Addition of a certain amount of A A monomer to P E A P U - P M M A A B C P s significantly increased the miscibility of the two components due to the formation of hydrogen bonds between carboxyl groups in A A and carbonyl groups in polyurethane segments. The A B C P s studied here have a two-phase structure with very small size domains. gd

gd

Acknowledgments The authors are very grateful to Jin Guiping, L i n H e , and Wang Deli for performing some experiments and preparing the manuscript.

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RECEIVED for review October

9, 1 9 9 1 .

ACCEPTED revised manuscript August

13, 1992.


Structure and Properties of AB Cross-Linked Polymers - American

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