Hydrolytic Degradation of Thermoplastic Copolyester Nanocomposites-Juniper Publishers
JUNIPER
PUBLISHERS- ACADEMIC JOURNAL OF POLYMER SCIENCE
Abstract
Thermoplastic copolyesters (TPE-Es) are multi-block
copolymers commonly incorporating hard (polyester) and soft (polyether)
blocks. Multifunctional epoxy-based chain extenders (TGDDM) are one of
the additives that serve to reverse molecular weight (MW)
damage caused by the process of hydrolysis of TPE-E. There are two
functional groups that are capable of additional reactions on the
terminal hydroxyl and carboxyl groups of TPE-E. Considering the hydrogen
bonded hard segments in hard domains are less susceptible to hydrolysis
compared to the non-hydrogen bonded equivalents, the phase separated
morphology of polyether-polyester copolymers based TGDDM also provides
to the long-term stability. The molecular weight of TPE-E was determined
by GPC and the thermal and tensile behaviors were investigated.
Keywords: Thermoplastic copolyester; Crosslinking; Hydrolysis; Mechanical properties
Abbrevations: TPE-Es: Thermoplastic Copolyesters; MW: Molecular Weight; TGDDM: Tetraglycidyl-4,4-Diamino-Diphenylmethane;
Introduction
Polyester hydrolysis produces hydrophilic acid
(carboxyl) end groups in the polymer, which increase the ability for the
polymer to absorb water because of the decrease in its surface
resistance and hydrophobicity [1-3].
Chain extenders remain necessary in improving the
molecular weight of TPE-E and, consequently, broaden their industrial
applications. Chain extenders help in the substitution of polyester
hydrolysis-caused molecular weight damage, leading to the production of a
polymer with desired melting strength and rheological properties. The
chain extenders function effectively, predominantly due to their
bi-functional groups that inherently react with polymer terminal groups.
Moreover, specific chain extenders,
such as tetraglycidyl-4,4-diamino-diphenylmethane (TGDDM), are vital in
the enhancement of TPE-E molecular weights. Consequently, TPE-E would
have improved rheological, mechanical, and thermal characteristics [4].
Hytrel® 3078 is a brand of TPE-E (copolyesters,
COPEs) under the trademark of DuPont. Hytrel 3078 nanocompsite prepared
by reactive extrusion-based masterbatch (HC1-C2), dry feed (HC2) of
organo-fluoromica combined with TGDDM, the post-addition of TGDDM
(HC1-T) and the direct-addition of TGDDM- organo-fluoromica (HT-C1) were
compared for their resistance to hydrolysis in aqueous buffer solutions
for 140 h solution at 100°C at pH 1 (Figure 1).
The enlargement of the MWD for the nanocomposites can also be indicated by the polydispersity index (PDI= Mw/Mn),
besides extremely high molecular weights above 2 × 106g/mol are also
obtained. These outcomes show modifier chain extension efficiency which
has an effect on the rheological characteristics of the material.
The PDI of the host polymer before and after
reactive modification via different processing routes (blank TPE-E,
HT-C1, HC2, HC1-C2 and HC1-T) are also given in Table 1. A significant
decrease in Mn of HC1-C2 nanocomposites occurred during a
masterbatch of reactive nanofiller in TPE-E. It is still expected
that there may be thermal and thermo-oxidative degradation
processes associated with reactive processing this TPE-E, which
are responsible for the molecular weight decrease in the melt
compounded TPE-E. The relatively low Mw, suggesting that
HC1-C2 is less crystalline than other modified nanocomposites,
and therefore it was not possible to obtain high molecular weight
TPE-E. Higher molecular weight may be obtained if the TGDDM
is post-added with TPE-E matrix for HC1-T. It was shown that
the Mn of the HC1-T was twice that of first adding of TGDDM into
TPE-E (HT-C1). This halving of molecular weight resulted in the
HT-C1 having a tensile strength of 35MPa, which was 4.9MPa
lower than that of the HC1-T (39.9MPa) (Figure 2). The postaddition
of TGDDM decreased the PDI of TPE-E nanocomposite.
This contrasts with the significant improvement in Mw of HC1-C2.
The high PDI value can also be attributed to higher branching
formation of TPE-E. Twin screw extrusion, which compounds
thermally-sensitive materials, yields a far greater shear energy
than single screw extrusion, which results in a loss of molecular
weight and a decrease in properties and performance [5].
The thermal and mechanical properties of polymers
are influenced by polydispersity of the hard segments. The
presence of many epoxy groups gives rise to a highly crosslinked
network within the nanocomposite. As seen in Figure
3, the addition of TGDDM in HT-C1, HC1-C2 and HC1-T clearly
shifted the exothermic heat of crystallisation peaks of the hard
segment to higher temperatures. The result of these studies
indicates that hard segments of uniform length crystalise faster
and better than hard segments of non-uniform length. However,
the hard segments represent a physical cross-link which report
elastomeric properties to the soft phase. The hard-soft segmental
solubility parameters and aligned hard and soft segments,
polydispersity and molecular weight, as well as the processing
and thermal history of the TPE-E host matrix are essential factor
to be considered to the tensile properties demonstrated by the
nanocomposites [6].
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