Hydrogen bonding in epoxy resin/poly(ε-caprolactone) blends

Abstract

SynopsisPoly(ccapro1actone) (PCL) of ca. 20,000 molecular weight is shown to be partially miscible with three aromatic-amine-cured epoxy resins. This conclusion is based on the depression of the epoxy T,, the effect on physical and mechanical properties, and the observation that a large proportion (40-55%) of the PCL ester groups are involved in hydrogen bonding. This miscibility behavior is compared to PCL blends with anhydride-cured epoxy resins, which appear to have a two-phase morphology. The different miscibilities are rationalized on the basis of the existence of functional groups (e. g., hydroxyl) in amine-cured epoxies which are capable of hydrogen bonding to the PCL ester groups. Anhydride-cured epoxy resins contain fewer potential hydrogen bonding sites. INTRODUCTIONEpoxy resins are used widely as the matrix component of high performance composite materials because of their stiffness, chemical resistance, and high-temperature stability.'v2 The fracture properties (fracture energy and tensile strength) of crosslinked epoxy resins, however, are of some concern, and a n extensive patent and scientific literature exists dealing with additives which modify these properties.s For example, the fracture toughness of an amine-cured epoxy resin can be increased by a factor of 3 by the incorporation of a carboxyl-terminated rubber: while other additives are described5s6 which can increase the tensile strength of amine-cured epoxy resins by at least 50%. As part of a larger study of the effect of additives on the properties of crosslinked epoxy resins, we describe here a study of epoxy residpoly(ecapro1actone) (PCL) blends and, in particular, the role of hydrogen bonding in influencing the miscibility of PCL with crosslinked epoxies.The majority of polymer-polymer blends are immiscible, i. e., the polymers do not mix on a molecular level.7 However, PCL has been shown to be miscible with a number of other polymers, particularly when an opportunity exists for hydrogen bonding or other polar interactions with the second p~lymer.~-l' The most extensive published study of epoxy resin/PCL blends is by Noshay and Robeson,12 who examined the miscibility of a range of anhydride-cured epoxy resins with PCL of different molecular weights and with different end groups. They concluded that above a critical molecular weight of PCL, the blends had a two-phase structure and that the * To whom correspondence should be addressed. PCL end groups reacted with the anhydride curing agent to produce a type of block copolymer. The purpose of the present work is to examine PCL blends with amine-cured epoxy resins and to compare their properties with anhydride-cured systems, because of the opportunity which exists for hydrogen bonding in amine-cured epoxy resins (reaction of an amine with an epoxide produces hydroxyl groups while reaction of an anhydride with an epoxide largely produces ester groups). EXPERIMENTALThe epoxy resins used for this study were of commercial origin and are classified here according to their major constituents: (a)...

Study of hydrogen bonding and thermal properties of polybenzoxazine and poly-(?-caprolactone) blends

“…The reduction of the hydrogen-bonded carbonyl may be in part associated with the occurrence of phase separation after gelation resulting in fewer opportunities for interaction. 16 This statement can be confirmed by DSC results, which will be discussed subsequently. The melting behavior of polycaprolactone is evident in the cured polybenzoxazine blends with a PCL concentration higher than 11 wt %, indicating the occurrence of the macrophase separation.…”

“…As seen in Figure 4, a much greater frequency shift (⌬) between the hydrogen-bonded and non-hydrogen-bonded carbonyl bands is exhibited in the blend with 6 wt % of PCL, indicating that the strength of the intermolecular hydrogen bonding between the carbonyl groups of PCL and the hydroxyl groups of poly(BA-a) is stiffer in the presence of lower amounts of the modifier. Moreover, the percentage of hydrogen bonding and the frequency shift (⌬) after curing isothermally at 180°C for 5 h are considerably lower than the maximum in all of the compositions, which is indicative of a much weaker hydrogen-bonding strength in the two-phase structure, whereas, in comparison with other reported data, the hydrogen-bonding strength in the PCL/benzoxazine blends (⌬ ϭ 22-25 cm Ϫ1 ) is slightly higher than in the PCL/ novolac (⌬ ϭ 20 cm Ϫ1 ), 22 PCL/epoxy (⌬ ϭ 17 cm Ϫ1 ), 16 and PCL/phenoxy blends (⌬ ϭ 15 cm Ϫ1 ). 20 It is suggested that a rather strong intermolecular hydrogen bond exists between PCL and poly(BA-a) even after the dramatic recovery resulting from the phase separation.…”

“…The fraction of the PCL carbonyl groups that are hydrogen-bonded to the benzoxazine resin is, therefore, a direct measure of how well the polymers are mixed on a molecular level. 16 The area of interest in the IR spectra of the blend is in the carbonyl-stretching region from 1760 to 1690 cm Ϫ1 . As presented in Figure 2, there are two bands in this region: one band centered at 1730 cm Ϫ1 , which is attributed to the characteristic peak of the non-hydrogen-bonded carbonyl, and the other is a relatively weak band between 1715 and 1705 cm Ϫ1 , which is assigned to the hydrogen-bonded carbonyl groups.…”

“…All spectra were curve-resolved to quantitatively analyze the hydrogen-bonding formation. Figure 3 shows the fraction of hydrogen-bonded carbonyl groups of PCL as a function of curing time that is calculated by the following equation 16 : (1) where A is the integrated intensity of hydrogenbonded carbonyl groups, and A F is the integrated intensity of free carbonyl groups. The absorptivities for the bands associated with the free and hydrogen-bonded carbonyl groups from PCL were taken as identical because the difference was relatively small in this particular system.…”

“…16,20 The bond length of all atoms between the two carbonyl groups on PCL [Scheme 3(a)] and the two hydroxyl groups on poly(BA-a) [Scheme 3(b)] were estimated. For a more accurate prediction, molecular models were energy-minimized, and the results are listed in Table III.…”