In the first step, formulation of polyether-based PU and its characterisations

Transcription

1 Chapter 8

2 Cliapter8 198 Despite the e~ergence of new shape memory alloys, the importance of shape memory polymer in both earth and space fields still dominates due to their excellent shape recovery characteristics. Thermo-responsive shape memory polymer find potential applications, particularly in space (deployable structures, antennas, smart hinges, rover wheels) and biomedical (sutures, bone screws, nails, plates, meshes, prosthetics, pumps, catheters, stents, orthopaedic braces, self-tightening knot) applications. Literature cites the development of a host of polymers belonging to the class of thermoplastics and a few thermosets exhibiting shape memory characteristics. From the practical application point ofview, the most important ones among them are polyurethanes. Polyurethane based shape memory foams have been employed in selfdeployable system for space applications. Polyurethanes are, by and large, interesting from the point of view of amenability for structural modification, compounding, copolymerisation etc: which opens immense avenues for tuning their shape memory properties. However, in open literature, one can rarely cite any systematic study directed towards correlating the shape memory properties to their structural, thermomechanical and morphological features. Among the thermosets, epoxy resins are the most versatile in view oftheir ease of synthesis, umpteen ways ofeffecting the curing, copolymerisation, compounding, composite processing etc. Infact, epoxies are the best suited for elastic memory composites with good strength characteristics and shape memory properties. Many reports refer to the use of proprietary shape memory epoxy formulations and composites derived thereof, for development of several selfdeployable systems for potential space applications. However, there are no details on their synthesis or processing aspects nor on the structural features leading to the manifestation of shape memory properties. Bigels that are inter-chain assembled through specific interaction provide alternate polymer systems with shape memory characteristics. Use of shape memory gels in extraction, absorption, actuator and drug delivery systems are of immense practical importance. A few bigels with thermoresponsive shape memory properties have been reported. There is immense scope for developing performance-improved shape memory gels based on the combinations of various cross-linked and thermoplastic polymers designed to have specific interchain secondary interactions. The ultimate property in shape memory polymer i.e. shape recovery, depends on the crystallinity of the polymer and/or the capability of the polymer to establish strong dipolar interactions in the 'frozen state'. This can be achieved by chosing a crystallisable segment with dipolar characteristics as the backbone. The choice converged on poly(tetramethylene oxide) possessing both these characteristics. The

3 Cliapter8 199 present work is aimed mainly on the synthesis and systematic investigations on various SMP systems based on PU and epoxy. The polyacrylate-ptmo based complexed gels are also included in the scope of investigations. Apart from synthesis and, characterisation of these polymer systems, studies have been extrapolated to matrix modification by way of reinforcements both at nano and macro levels. In the first step, formulation of polyether-based PU and its characterisations were studied in detail. PUs based on poly(tetramethylene oxide), butane diol and tolylene diisocyanate were synthesized by varying the hard segment content between 33 and 85% by a two-step process. An increase in hard segment content resulted in a diminution in molecular weight and the apparent T trans. Tensile strength and elongation of the PU decreased with hard segment content, while modulus increased. FTIR analysis confirmed the presence of urethane linkage. Spectroscopic analyses also provided evidence for increase in degree ofphase segregation ofurethane by way ofhbonding on augmenting the hard segment content. The XRD analysis showed PUs to be possessing both defined and diffused crystallinity at ambient temperatures. From the x ray diffraction studies, it was concluded that crystallinity increased with PTMO concentration and on lowering the temperature. Mophological analysis exhibited the crystalline phase of soft segment in PTMO-rich compositions. Storage modulus, EglEr ratio as well as the shape recovery property showed an increasing trend with the hard segment content. PU system with hard segment content greater than 73% exhibited more than 85% shape recovery. Shape recovery as high as 96% was observed for the system with maximum hard segment-content (85%) in the series. By the control of hard-soft segment combination in PU architecture, it was possible to realize SMP of desired recovery characteristics. As an optimum shape recovery of 96% was obt:'\ined for PU with 85-wt% hard segment content, the PU system of this composition was used for nanoclay reinforcement studies. Nanoclay (Claosite 25A) reinforcement was varied from 1 5wt%. Increase in clay content resulted in an increc'.se in transition. Modulus and tensile strength increased proportional to nanoclay content while percentage elongation decreased. XRD studies showed the presence of intercalated structure of clay in the PU matrix, which was also confirmed by TEM analysis. Modulus ratio showed a decreasing trend with nanoclay content. Highest shape recovery of 92 percentage was observed for PU system with 1wt-percentage clay content. Nanoclay modification led to a decreased shape recovery characteristics vis-a.-vis neat PU. This was attributed to a reduction in EglEr ratio, as nanoclay reinforced the rubbery polymer more than the glassy state. Thus, nanoclay modification provided a means for obtaining shape

4 Cliapter8 200 memory PU with goqd rigidity and increased mechanical properties, with marginal penalty on shape memory properties. As nanomodification did not improve the shape memory properties, a chemical modification of PU backbone was envisaged for achieving the same. Thus, the influence ofincorporation ofoxazolidone moieties on PU backbone on the mechanical, thermo-mechanical and shape recovery characteristics was studied. Oxazolidoneincorporation on PU matrix (with varying hard-soft segment contents) was achieved by reaction of a diepoxy resin with isocyanate-telechelic PTMO. The latter was synthesized from OH-telechelic PTMO by reaction with TDI. Hard segment content was varied between 11 and 26-weight percentage. Unlike nanoclay modification, structural modification by way of oxazolidone-groups resulted in diminution in (Ttrans), and the mechanical properties of the resultant PUOs. Maximum Ttrans of 50 C was observed for highest PTMO-content of 89-weight percentage. The derived PUO were insoluble in THF, DMF etc. implying their intractable nature. FTIR analysis confirmed the presence ofurethane and oxazolidone groups. Like in the case ofpu, the PUOs also possessed diffused crystallites at ambient temperature and tendency to crystallize on cooling. SEM analysis showed crystalline domains of PTMO. An increasing trend of crystallization and EglEr ratio with increase in the oxazolidone-content in the polymers was evidenced by dynamic mechanical analysis. Cyclic tensile test evaluation for the shape memory property confirmed excellent shape recovery characteristics for the oxazolidone-modified PU. The shape recovery increased with oxazolidone concentration as these groups increased crystallization tendency of PTMO segments. All the PUO exhibited more than 95 percentage shape recovery and 99 percentage shape fixity. Their shape memory properties are superior to those of pure PU, though the mechanical properties are adversely affected Thus, appropriate structural modification is a better means for conferring desired shape memory property. But it is desirable that the resultant polymer be tractable. Elastic memory composites, derived frori the shape memory polymers are required as structural components for various self-deployable systems, particularly for space applications. This also warrants that the polymer possesses the characteristics required of a good matrix in polymer composites. So far, with the exceptation of cyanate ester, epoxies are the best known matrices in polymer composites with the added advantage of ease of processability. Hence, it was of interest to molecularly design a shape memory epoxy resin. This was realized by synthesising an aminetelechelic PTMO and using the same to cure various epoxy resins in different proportions. The amine telechelic PTMO was derived from hydroxyl telechelic PTMO

5 Cliapter8 201 by a two-step reaction involving transformation to isocyanate terminated polymer and its eventual end-capping with an aromatic diamine. During curing, molar ratios ofthe amine and epoxy reactants were varied to get epoxy-polymer of varying properties. Three different epoxies (GY-250, EPN and TRI) were used to study the shape memory characteristics. During the synthesis of the epoxy-amine system, a spurious reaction with the associated formation of carbodiimide groups in minor quantitative was noticed. Mechanism for formation of these groups was elucidated through studies on model compounds. These groups originated from the reaction of the urea groups (present in amine telechelic PTMO) with the epoxy groups. From DSC and rheological analyses, the cure reaction of the polymer system was optimized. A first order kinetics was observed for epoxy: amine reaction from DSC investigation. The isothermal cure prediction from DSC kinetic data matched with the actual cure behaviour as observed in rheokinetic analysis. This study led to cure optimization. Increasing epoxy monomer content resulted in progression in T trans. Tensile strength and initial modulus ofthe GY 250 epoxy system increased whereas the percentage elongation decreased with epoxy monomer. EPN and triepoxy systems were too brittle to determine the mechanical properties. From XRD analysis, it was confirmed that the epoxy polymer possessed broader crystal dispersion at room temperature, and crystallites developed on cooling the system to ~10-15 C in the glassy state. DMTA of GY systems exhibited a decreasing trend in storage modulus with epoxy-content. EPN exhibited higher storage modulus compared to the GY and TRI system for the same amine: epoxy composition (at temperatures where PTMO segment is in the crystalline state). TRI possessed comparatively low glassy modulus with respect to all other systems. For a given epoxy system, increasing PTMO concentration was conducive to enhancing the shape recovery with a marginal penalty in the recovery time. All the epoxy-based systems exhibited more than 85% shape recovery. About 99% shape recovery was observed for GY system with maximum PTMO concentration. The study showed that diepoxy based system with a good PTMO content is the best suited one for shape memory applications. Composites of the above epoxy based shape memory polymers with carbon fabric (T-300) were processed. All the three epoxies (GY-250, EPN and TRI) were used for the development of composites in 1: 1 amine-epoxy ratio. For GY-250 epoxy system, the carbon fibre was varied from 20 to 50wt% and it showed increased transition temperature with carbon-content. Composites with carbon: shape memory resin ratio of 50:50 only was used for the development of EPN- and TRI-based composites. Comparing the three different epoxy systems (with carbon: shape memory

6 Cliapter8 202 resin ratio of50:50 co.mpositions), the composite with GY-250 exhibited the maximum T g value of 119 C. Flexural properties of GY;250 composites increased with carbon fabric content, and attained an optimum value for 40wt% carbon fabric content. EPN and TRI systems exhibited comparatively decreased flexural properties due to enhanced crosslinking. The GY series composites with 60% resin content exhibited higher storage modulus. EPN composite system possessed higher storage modulus than TRI system. All the developed composites exhibited more than 90% shape recovery. The GY series of composites exhibited the maximum shape recovery of 97%. The shape recovery remains constant with resin content. The recovery time was directly proportional to the epoxy resin-content of the composites. The composites have the required strength and shape memory properties for possible use in the development of thermo-responsive self-deployable systems. Stimuli responsive gels have attained immense attention due to their potential applications in the field as smart materials, including in controlled drug-delivery systems, culture substrates, chemical valves and gentle actuators. Hence, it was of interest to develop shape memory polymer gels. Polymer gels ofpoly(acrylic acid-coacrylonitrile) were synthesized by varying the compositions of AA and AN in the prepolymer, and blended with PTMO to get PTMO/poly(AA-co-AN) complexed gels. The complexation occurred mainly through H-bonding of carboxyl group of acrylic acid with the ether group of PTMO. FTIR analysis gave evidence for the presence of H-bonding in PTMO/poly(AA-co-AN) gels. The swelling ratio of PTMO/poly(AA-co AN) gels increased with acrylic acid monomer in the crosslinked polymer. With enhancement in the acrylic acid-content, Tg of the complexed gels diminished. The modulus ratio (EglEr) of the PTMO/poly(AA-co-AN) gels showed an increasing trend with increase in the acrylic acid content, which was cpnducive to better shape memory and shape recovery properties. All PTMO/poly(AA-co-AN) gels exhibited more than 93% shape recovery. The complex with highest EglEr ratio, exhibited the highest shape recovery of 99%, which resulted from enhanced H-bonding interactions. About 99% shape recovery with a modulus ratio of 168 and tan8 magnitude has been reported for H-bonded complex comprised of poly (acrylic acid-comethylmethacrylate) networks and polyethylene glycol. The present system exhibited 99% shape recovery in 60 seconds with an elastic modulus ratio and tan8 maxima of 800 and 1.4 magnitudes. Tan8 maxima>1 implies high energy absorbing capacity, which could find potential applications of the developed system in vibration control area too.

7 Cliapter8 203 Future Scope of the Work The studies in this thesis show the prospects ofthree SMP system. There is scope for changing the switching segment from PTMO to other systems like polycaprolactone, polyethylene oxide with good crystallisable characteristics. It is desirable to select the nature and molecular weight ofthe switching segment such that it crystallizes at ambient temperature and melts at temperature in the vicinity of IOOcC. For this investigation, PTMO of different molecular weight will be ofinterest. Conferring shape memory property to thermosetting polymers is ideal as the thermosets are suited for primary moulding and for deriving EMC. It will be of interest to investigate cyanate ester systems with a known edge over the epoxies. Another interesting system is toughened bismaleimides. All these call for challenges in various synthesis strategies. Nano modification ofknown SMP system is another area for future research. Although the present thesis has made a humble attempt in selected cases with nanoclay, other nanomodifiers like CNT, POSS can be explored for their effect on the properties ofknown SMPs.

8 C/iQpter8 204 As a whole" the research work has led to the following conclusions (i) (ii) (iii) (iv) Each of the developed system is suited for a specific application depending on their consistency, mechanical properties, and shape recovery characteristics. Shape memory PU with tailorable mechanical properties and shape memory characteristics can be designed by varying the soft/hard segment ratio, keeping PTMO as the switching segment. Shape recovery as high as 96% is achievable for such systems. The system being tractable can be effectively used for thermo-responsive shape memory foam for related applications in space. However, the shape recovery of the foam as a system has independently to be investigated. Shape memory polymer with increased strength and modulus can be attained by nanoclay modification on the PU matrix. A shape recovery of ~ I00% is achievable by way of structural modification of PU. Structural modification with respect to oxazolidone groups is one such approach. However, the insolubility of the polymer is a major concern for their secondary moulding. Epoxy-based shape memory polymer with good shape recovery properties can be worked out by varying the nature and concentration of epoxy monomer to get different softlhard segments suited for shape recovery properties. About 99% shape recovery is feasible for such systems. Elastic memory composites of epoxy shape memory resin can be designed using various epoxies while maintaining the PTMO as the backbone. As high as 97% shape recovery is obtainable for such systems. However, the attainable strain levels are less. The developed systems are being evb.1uated for their use in smart hinges for space applications. (v) Polymer gels of acrylate-based system with tunable shape recovery characteristics can be realized by choosing appropriate monomer compositions for the gels for complexation with PTMO. A higher percentage of (as much as 99%) shape recovery is attainable for such systems.