A study on co-continous morphologies in polymer blends: formaion, stability and properties

Abstract

A Study on Co-continuous Morphologies in Polymer Blends.

Formation, Stability and Properties.

Harm Veenstra

Polymer Materials and Engineering, Delft University of Technology, Delft, The Netherlands

Summary:

The aim of this study is to obtain a better understanding of the formation, stability and properties of co-continuous morphologies in polymer blends. Special attention is paid to blends where one of the components is a thermoplastic elastomer (TPE). However, also blends of ordinary pseudo-plastics are considered. TPEs are block copolymers where the blocks are phase separated, resulting in physical crosslinks. To explain the formation and stability of co-continuous blends with TPEs an exact knowledge of the presence of physical crosslinks and related rheological properties of the block copolymers is essential.
Therefore the order-disorder transition (ODT), where a phase separated melt changes into a single-phase melt, and rheology of a poly(ether-ester) multiblock copolymer are described in Chapter 2. Rheological measurements reveal a wide ODT (207°C-224°C) upon heating which coincides perfectly with the melting range. From this coincidence together with the Maxwell fluid behaviour directly following the ODT it is concluded that melting leads to a one phase liquid and that no separate segmental mixing transition occurs. The wide ODT implies a large processing window over which the rheological properties change from highly elastic, with a distinct yield stress, to normal pseudoplastic.
In Chapter 3 the rheology and ODT of SEBS block copolymers are described. It is shown that the SEBS 1657 shows an ODT at 212°C. However, no distinct yield stress was found at temperatures where a phase separated melt is present, i.e. below the ODT. The SEBS 1652 block copolymer shows no ODT, meaning that the block copolymer is phase separated at all temperatures measured. The SEBS 1652 therefore shows highly elastic rheological properties, with a distinct yield stress, at all processing temperatures.
The formation and stability of co-continuous blends composed of polystyrene and poly(ether-ester) is described in Chapter 4. It is shown that stable co-continuous morphologies are obtained over a wide composition range (30-80 vol%) when the polymers are blended below the block copolymer's ODT. This range decreases with increasing processing temperature and becomes limited when both polymers show pseudo-plastic behaviour. The condition for the formation of co-continuous morphologies, especially at low volume fractions, is the existence of stable interconnected elongated structures, which do not show breakup or retraction. Breakup and retraction experiments on poly(ether-ester) fibers imbedded in a polystyrene matrix make clear that these mechanisms can be severely limited or even stopped. The limiting force for preventing breakup is the yield stress of the block copolymer, as measured in chapter 2. The non-breakup behaviour is quantitatively explained using an expression that is based on the assumption that a distortion is unable to grow if the pressure difference, which is generated in the thread by the different radii of curvature, is smaller than the yield stress.
The formation and stability of co-continuous morphologies in blends with SEBS block copolymers were described in Chapter 5. It is shown that stable co-continuous blends can be obtained over a wide composition range when the block copolymers are blended with polypropylene at temperatures where the block copolymers are microphase separated. Blending the same block copolymers with polymethyl methacrylate or polyoxymethylene, leading to blends with much higher interfacial tensions, results in a much smaller composition range of co-continuous morphologies than was found in the PP/SEBS blends, whatever the processing temperature. It is demonstrated that breakup and retraction can be severely limited or even stopped at lower blending temperatures, therefore fulfilling the condition for stability of co-continuous morphologies. The (non-)breakup or (non-)retraction behaviour of elongated structures strongly depends on a complex combination of parameters, including phase size, yield stress and interfacial tension. The intrinsic instability of co-continuous morphologies is still one of the main problems. Annealing co-continuous morphologies at temperatures where the polymers are in the liquid state does not only affect the dimensions of the phase domains but also the range of volume fractions where co-continuous morphologies are found. Therefore, it is difficult to control these morphologies during further processing such as compression and injection moulding.
To obtain a better knowledge of the instability of co-continuous blends in for example compression moulding, Chapter 6 deals with the coarsening of co-continuous morphologies in polymer blends in quiescent conditions. It is shown that co-continuous morphologies of 50/50 wt% stay co-continuous upon annealing but show a linear increase in phase size with time. The 30/70 wt% blends break up into droplet/matrix morphologies and show an increase in phase size in the first stage of the coarsening process. A model has been proposed, using only the interfacial tension and viscosities of both components, to predict the growth rate of coarsening. The model shows excellent agreement with the rates found experimentally. The coarsening process in blends with TPEs is severely slowed down or even stopped when physical crosslinks are present in the thermoplastic elastomers. The driving force for coarsening is too small to overcome the stabilizing force due to physical crosslinks or a yield stress.
In Chapter 7 the stability of co-continuous blends composed of polystyrene and poly(ether-ester) in shear flow is described. It is shown that these blends are not stable under shear and break up into droplet/matrix morphologies. The capillary number at these flow conditions is too small to maintain stable extended structures that are necessary for a co-continuous morphology. Co-continuity at such flow conditions is limited to the point of phase inversion. The knowledge of the formation and stability of co-continuous morphologies, as obtained from abovementioned chapters enabled us to adapt the processing conditions in such a way that at a single volume fraction, either co-continuous or dispersed morphologies could be obtained. Therefore the mechanical properties of co-continuous blends could be compared to those of dispersed morphologies.
It is shown in Chapter 8 that the elastic moduli of co-continuous blends are significantly higher than the moduli of the dispersed blends but no significant difference in tensile strength or impact strength was found when co-continuous blends are compared to blends with a droplet/matrix morphology. A model is proposed depicting the basic element of a co-continuous structure as three orthogonal bars of one component embedded in a unit cube where the remaining volume is occupied by the other component. This model is shown to predict the moduli of polymer blends with co-continuous morphologies over the complete composition range.