Water in urethane acrylates—II. Dynamic mechanical properties of wet urethane acrylate networks

Eur. Polym. J. Vol. 29, No. 11, pp. 146%1471, 1993 Printed in Great Britain. All rights reserved

0014-3057/93 $6.00+0.00 Copyright © 1993 Pergamon Press Ltd

WATER IN U R E T H A N E ACRYLATES--II. D Y N A M I C MECHANICAL PROPERTIES OF WET URETHANE ACRYLATE NETWORKS A. B. CLAYTON, l* P. E. M. ALLEN 1 and D. R. G. WILL1AMS2~ Departments of ~Physical and Inorganic Chemistry and 2Cbemical Engineering, University of Adelaide, GPO Box 498, Adelaide, S.A. 5001, Australia (Received 16 December 1992)

Abstract--The effects of absorbed water on the mechanical properties of a series of urethane acrylate polymers were determined by DMTA analysis. The dynamic mechanical behaviour of the wet polymers differed from that of the dry networks, depending on the soft segment type and molecular weight. The results are interpreted in terms of the distribution of water within the polymer, with water poorly associated with the polymer forming freezable clusters or domains.

INTRODUCTION In the previous paper [1] we described the sorption and desorption of water in urethane acrylate networks prepared from prepolymers consisting of polyether [polytetramethylene oxide (PTMO)] and polyester Loolycaprolactone (PCL)] diols of various molecular weights end-capped with toluene 2,4 diisocyanate (TDI)/2-hydroxyethyl acrylate (HEA) groups (Fig. 1). In this paper we are concerned with the changes in dynamic mechanical response as water is introduced into the polymer sample, EXPERIMENTAL PROCEDURES Sample synthesis, polymerization and hydration procedures have been described previously [1]. The equilibrium water content, EWC, is the percentage by weight relative to the wet sample. The numbers of repeat units, n, in the polymer diols were ca 9, 14, 20 and 28 for the PTMO 650, 1000, 2000 and 2900 respectively. PCL 1250, 2000 and 3000 diols contained 1l, 18 and 26 repeat units respectively, After casting, dynamic mechanical thermal analysis was carried out on 2 x 8 x 40 mm samples cut from the polymer sheets. A Polymer Labs DMTA MkII was used in the bending mode, with the sample clamped in a double cantilever geometry: strain applied to the samples kept to < 1% of sample thickness in order to avoid any non-linear effects [2]. Samples were usually scanned between - 100 and + 100°C at 3°C/min using a N 2 gas purge, with rubbery samples being reclamped at low temperature to ensure adequate mechanical contact through the scan. RESULTS AND DISCUSSION The dynamic mechanical properties of wet and dry urethane acrylate homopolymers, together with sample EWCs, are shown in Table I. DMTA runs of the saturated urethane acrylate polymers showed three distinct types of dynamic mechanical response *Present address: Department of Chemistry, Heriot-Watt University, Edinburgh, Scotland. tTo whom all correspondence should be addressed,

compared to the dry polymer. For the TET650, TESI250 and TdiET650 polymers, decreases in Tg consistent with water plasticization were seen. A second group of polymers (TET1000, TES2000, TES3000 and TdiET1000) showed little change in Tg with slight increases in tan 6 at the glass transition for the saturated polymer. A final category (TET2000, TET2900 and TdiET2000) showed increases in Tg, together with large decreases in the loss peak height. The three types of dynamic mechanical response are exemplified in the tan 6-temperature plots for TET650 (Fig. 2), TES2000 (Fig. 3) and TdiET2000 (Fig. 4). The Fox equation [3] may be used to predict Tg of wet polymers: 1 T~

Wp q- Ww Tg(p) Tg(w)

( 1)

where Tg(p) and Ts(w ) refer to polymer and water respectively, and W is the weight fraction of the two components. Equation (1) was used to calculate Tgs for those samples in which a decrease in Tg was observed (i.e. plasticization was apparent), with Tg of water being set at 134 K [4] (Table 2). Calculated Tgs for TET650 and TdiET650 showed good agreement with those obtained experimentally, while the calculated Ts for the TESI250 sample was 7°C high. Insignificant water plasticization occurred in several of the wet urethane acrylate polymers. Despite their low EWCs, absorption of comparable amounts ( I - 3 % by weight) of water has been found to cause much larger reductions in the Tg of epoxy resins [5-7]. Several possible mechanisms for plasticization of crosslinked polymer networks have been proposed. One mechanism involves the replacement of polymet-polymer hydrogen-bonds between polar functional groups [8] with water-polymer bonds, effectively reducing the network crosslink density. Other workers [9], however, maintain that the plasticizing effect of water in epoxy resins can be explained by an increase in free volume decreasing the activation energy for the Ts process.

1467

1468

A.B. CLAYTONet al.

Agreement of saturated polymer Tgs with the Fox equation implies that water is intimately associated with motional units in the polymer, rather than being isolated in droplets or clusters. The lack of any T~ shift for the wet TET1000 polymer relative to that observed for the saturated TES1250 sample also suggests that the nature of the soft segment has an effect on water distribution in the polymer, A number of studies on the distribution of water in hydrogel networks suggest [10-16] that the water in the hydrogel exists in a continuum of states between two extremes. Water strongly associated with the polymer network is referred to as "bound" or non-freezing water; water unaffected by the poly-

meric environment is referred to as "free" or freezing water. Distribution of water in different environments may also occur in polymers with much lower water contents than saturated hydrogels: Moy and Karasz [17] studied plasticization in wet epoxy resins and observed no water melting endotherms. Such behaviour may be explained either by water in the polymer being present in droplets small enough to supercool, thus giving no melting endotherm, or by water being strongly associated with polar groups in the polymer forming a non-freezing phase. The state of water in the polymer may also influence mechanical properties. DSC studies on PHEMA

TET series

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O((CH2)40)nTDI - Fig. I. Structures and designations for urethane acrylate prepolymers.

HEA

W a t e r in u r e t h a n e a c r y l a t e s - - l I Table 1. Dynamic mechanical properties of dry and saturated urethane acrylate polymers Polymer

EWC(%)

TET650

0 3.1 0 2.8 0 2.0

TET1000 TET2000

rET2900

0 2.0

TES1250

0 1.8 0 1.6 0 1.5 0 3 0 2.5 0 2.1

TES2000 TES3000 TdiET650 TdiETI000 TdiET2000

0,462 0.516 0.329 0.430 0.302 0.350 0.263 0.307 0.231 0.286 0.371 0.373 0.405 0.413 0.387 0.363 0.498 0.542 0.428 0.494 0.534 0.179

54 52 82 68 88* --

6.972 6.921 7.030 6.764 7.392 7.100

131 * --

7.368 7.314

70 56 54 46 44 48 60 42 50 36 25 74

6.854 7.058 6.975 6.895 7.493 7.768 6.533 6.714 6.981 6.813 7.935 7.424

Ts (glass transition temperature) has been taken from the tan 3-temperature plot. Peak height and width at half height (Wi, 2) refer to the tan 6 glass transition peak. Log storage modulus values are given for Tg + 4 0 C .

• Denotes asymmetricpeak.

---, denotes peak insufficiently resolved for W, 2 to be determined.

[11] showed that, above 20% EWC, water existed as a "bulk-like" phase capable of freezing in a cooled polymer. Torsion pendulum data on PHEMA hydrated to low water contents [18] showed initial plasticization of the glass transition in accordance with the Fox equation. The Tg found for a fully hydrated p H E M A sample (0°C), however, was 48°C higher than predicted, This change in dynamic mechanical behaviour may be due to the formation of ice in the polymer. Kolarik and Janacek [19] attributed the sudden decrease in log

0.5

0.5"

Log E ' (Tg+4@'C)

T~ (°C) tan 6 (Tg) W~,2("C) + 35 +23 + 13 + 14 50 --32 +34 - 50 -32 -4 +8 -4 - 26 -26 - 37 37 - 6 -15 -32 34 -54 - 34

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and wet (0)

(storage modulus) in hydrated pHEMA samples after 0 ° C t o the melting of crystalline w a t e r . The greatest changes in dynamic mechanical response with water uptake are in those urethane acrylate polymers with PTMO soft segment molecular weights >_-2000. Tan 6 (Tg) increased for the wet TET2000 polymer while the Tg itself showed an 18°C temperature increase. Saturated TET2900 showed both a decrease in tan 6 (Tg) and an increase in Tg from the dry polymer, possibly indicating a greater degree of ice formation in the polymer compared to TET2000, either as the sample was cooled to the DMTA run start temperature or through the run itself. The most drastic effects were observed for the sample with the lowest crosslink density of any

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1470

A.B. CLAYTONet al.

Table 2. Resultsof the Fox equation for water plasticizedurethane acrylates. The Ts of water w a s s e t at 134 K Experimental Fox equation Polymer Mass% water Tg(K) rg (K) TET650 3.1 296 297 TESI250 1.8 269 275 TdiET650 3.0 258 260 hydrated sample considered viz. the TdiET2000 polymer: presumably ice formation in this sample was extensive and effectively dampened the motions of the PTMO chains, From the results obtained here, it is possible to classify water in these polymers as being in three states: (1) Intimately associated with the motional units, plasticizing glass transition temperatures and increasing polymer free volume. (2) Moderately associated with motionai units --increasing free volume, but not influencing Tg. (3) Poorly associated with motional units and forming clusters or droplets of differing sizes, which in some polymers are large enough to permit ice formation either on cooling or during thermal scan. The increasingly hydrophobic nature of the PTMO polymer chains with higher molecular weight appears to lead to decreased plasticization of polymer glass transitions, but ultimately to water clustering, which may result in the formation of a water phase capable of freezing. The decreased hydrophobicity of the poly(caprolactone) diol soft segment compared to PTMO may result in water in TPCL polymers not forming isolated clusters, but being moderately associated with PCL soft links, although clearly the affinity of water for the PCL soft links cannot be great--as evidenced by the low water uptake in the TES polymer series, L o w temperature Tdi~ p e a k s

In addition to the low temperature transition at 120°C and the glass transition peak, another peak develops on water sorption into the urethane acrylates. This peak is termed Tdil, and appears to be present to a limited extent in the TET650 homopolymer. Copolymers of epoxy bis G M A and tetraethylene glycol dimethacrylate showed a peak at -110°C (1 Hz) [20] which increased in magnitude for higher tetraEGDMA contents. Dynamic mechanical plots of the subambient temperature region in saturated and dry TET650 samples are shown in Fig. 5: these plots were typical of those found for the other urethane acrylates• Ta, 1 peaks appeared between - 8 0 and - 7 0 ° C in wet TES and TdiET polymers. Koshiba et al.[21] observed a similar peak at - 6 0 ° C in an isophorone diisocyanate/PTMOl000 based polymer despite storing samples over silica gel in a desiccator. The higher temperature of this peak may be due to the higher measurement frequency (110 Hz). In common with results obtained [22] for a homologous ethylene glycol dimethacrylate series the T~, peak decreased as the Tdij peak appeared in saturated samples,

The growth of Tdi~ peaks in linear methacrylates has been ascribed to the interaction of the penetrant molecules with ester side-groups in the methacrylate. Disruption of polar polymer-polymer interactions on solvent sorption and subsequent replacement by solvent-polymer interactions has been invoked [23] to account for the observed increase in side-chain mobility. The temperature of the ~ peak in pHEMA networks has been found to be independent of the length of the side-chains [24] confirming that the 7 peak is due to localized internal motions in the side-chains. The urethane group undoubtedly can hydrogen-bond to water absorbed into the polymer [25, 26] and, since the urethane groups occur in the side-chain, localized side-chain motions are affected on sorption of water into the network. CONCLUSIONS Uniform effects of water sorption on homopolymer glass transitions in saturated urethane acrylates were not observed, with some saturated polymers showing Tg decreases readily explicable in terms of water plasticization, while others showed no change in dynamic mechanical properties relative to the dry polymer or increases in T~ accompanied by decreases in the loss peak height. The observed changes in polymer dynamic mechanical properties when equilibrated in water may be due to water residing in different environments within the network. While the water uptake in the urethane acrylates is low (1.5-3.1% EWC) compared to that in p H E M A networks (37-39%), similar partitioning of water in a range of environments may occur, with water directly bonded to the urethane NH group and associated with carbonyl groups being in the nonfreezing water category. Freezing water probably forms more extensively in networks where the hydrophobicity of the polymer chains promotes water clustering: TES3000 containing PCL soft segments is unaffected by water sorption

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Water in urethane acrylates--II while the a m o r p h o u s glass transition o f saturated TET2900 increased by nearly 20°C, with a concomit a n t decrease in tan ~ (Tg).

Acknowledgements~n¢ of the authors (ABC) thanks Telecom Research Laboratories and the Australian Postgraduate Research Award scheme for funding of this project.

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