POLYMER WITH IMPROVED FRACTURE TOUGHNESS

Abstract

A polymer comprising a structural unit of formula (I). wherein each X is independently a C.sub.1-10 hydrocarbylene group optionally comprising 1 to 6 heteroatoms: Y is a C.sub.2-20 hydrocarbylene group: Z is a C.sub.1-20 hydrocarbylene group optionally comprising 1 to 6 heteroatoms or a linking group comprising one or more alkylene glycol or ethylene diamine groups: n is 12 to 100; and m is 2 to 1000.

##STR00001##

Claims

1. A polymer comprising a structural unit of formula (I): ##STR00012## wherein each X is independently a C.sub.1-10 hydrocarbylene group optionally comprising 1 to 6 heteroatoms; Y is a C.sub.2-20 hydrocarbylene group; Z is a C.sub.1-20 hydrocarbylene group optionally comprising 1 to 6 heteroatoms or a linking group comprising one or more alkylene glycol or ethylene diamine groups; n is 12 to 100; and m is 2 to 1000.

2. The polymer according to claim 1, wherein each X is independently a C1-6 hydrocarbylene group, preferably a C1-4 hydrocarbylene group, such as a C3 hydrocarbylene group.

3. The polymer according to claim 1 or claim 2, wherein each X is independently a C1-6 alkylene group, such as n-propylene.

4. The polymer according to any preceding claim, wherein Y is a C6-20 hydrocarbylene group, preferably a C6-12 hydrocarbylene group.

5. The polymer according to any preceding claim, wherein Y is a C6-12 cycloalkylene group, preferably a branched C6-C12 cycloalkylene group, such as 1,1,3,3-tetramethylcyclohexylene.

6. The polymer according to any preceding claim, wherein Z is
-L1-(OCH.sub.2CH.sub.2O).sub.q-L2-
-L1-(NHCH.sub.2CH.sub.2NH).sub.q-L2- wherein q is 1 to 10 and L1 and L2 are independently a C1-4 alkylene chain; or
CH.sub.2(CH.sub.2).sub.wCH.sub.2 where w is 0-10. wherein q is 1 to 10 and L1 and L2 are independently C1-6 alkylene groups, such as
CH.sub.2CH.sub.2OCH.sub.2CH.sub.2OCH.sub.2CH.sub.2.

7. The polymer according to claim 6 wherein q is 1 and L1 and L2 are ethylene.

8. The polymer according to claims 1 to 7 wherein Z comprises an ether or an amino group, especially a polyether functional group.

9. The polymer according to any preceding claim, having a Young's modulus of greater than 1 MPa, preferably greater than 2 MPa, more preferably greater than 3 MPa, such as greater than 10 MPa.

10. The polymer according to any preceding claim, having a fracture toughness of greater than 5 kJ/m.sup.2, preferably greater than 10 KJ/m.sup.2, such as greater than 15 kJ/m.sup.2.

11. The polymer according to any preceding claim, being an elastomer.

12. The polymer according to any preceding claim, wherein the polymer is a thermoplastic.

13. A polymer composition comprising the polymer according to any preceding claim, preferably an unfilled polymer composition.

14. The polymer according to any preceding claim wherein the molecular weight of the siloxy repeating unit (i.e. the repeating unit within the curved brackets) is preferably at least 1000, such as at least 1200, especially at least 1500.

15. The polymer according to any preceding claim having a melting point measured by DSC of at least 50? C.

16. Use of a polymer or polymer composition according to any preceding claim in the manufacture of films, wearable electronics, flexible electronics, soft robots, vehicles, in construction, and in soft-lithography.

17. An article, such as a film, wearable electronic device, flexible electronic device, soft robot or vehicle, comprising the polymer or polymer composition according to any of claims 1 to 15.

18. A method of preparing a polymer comprising a repeating unit of formula (I) ##STR00013## comprising the steps of: (i) reacting a compound of formula (A) with a compound of formula (B) to form an intermediate of formula (C); (ii) reacting said intermediate of formula (C) with a chain extender of formula (D) ##STR00014## wherein X, Y, Z and n are as defined for the polymer according to formula (I) in claim 1.

19. The method of claim 18, wherein the chain extender of formula (D) is 1,2-bis(2-aminoethoxy)ethane or triethylenetetramine.

20. A polymer comprising a repeating unit of formula (II)
-[PDMS-Q]- (II) wherein PDMS is a polydimethylsiloxane having an Mw of at least 1200 and Q is a group comprising at least two urea (NHCONH) groups, and at least one alkylene glycol or alkylene diamine group in the backbone of the polymer.

21. The polymer according to claim 20, wherein Q comprises at least 4 urea groups.

22. The polymer according to claim 20 or 21 wherein Q comprises the group
-L1-(OCH.sub.2CH.sub.2O).sub.q-L2- or -L1-(NHCH.sub.2CH.sub.2NH).sub.q-L2- wherein q is 1 to 10 and L1 and L2 are independently a C1-4 alkylene chain.

23. The polymer according to any of claims 20 to 22, wherein Q comprises the structural unit ##STR00015##

Description

BRIEF DESCRIPTION OF THE FIGURES

[0119] FIG. 1: FIG. 1a shows stress-strain curves for notched and unnotched samples of the polymer according to the present invention. FIG. 1b shows the stress strain curves for unnotched samples of the polymer in the low strain region.

[0120] FIG. 2: FIG. 2 shows SEM images of pre-cracked specimen of PDUE5000 during loading and unloading. In FIGS. 2a-c the pre-crack is opening and blunting in the initial stretching (?d=0.1-3.0 mm, scale bar: 1.00 mm). In FIGS. 2d and 2e a fibre-like flaw generates at the edge of the specimen (scale bar: 1.00 mm and 400 ?um respectively). In FIGS. 2f-h, more flaws generate and propagate along the direction parallel to the stretching (scale bar: 1.00, 1.00 and 2.00 mm respectively). In FIGS. 2i and 2j the cracks are observed clearly during unloading (scale bar: 1.00 mm and 200 ?m respectively). In FIGS. 2k and 2l, crack deflection and branching are observed in the unloading state. The cracks turn around rather than propagate in the original direction (scale bar: 1.00 mm and 100 ?m respectively).

[0121] FIG. 3: FIG. 3 shows the thermogravimetic analysis (TGA) curve for PDUE5000 of the polymer.

[0122] FIG. 4. FIG. 4a shows a theoretical depicted of hydrogen bonding in the claimed molecules. In FIG. 4(b), PDUE molecule consists of flexible chains (blue) and eightfold hydrogen-bonding motifs, forming very homogeneous networks. In FIG. 4(c) a toughness-modulus diagram of PDUE (black stars) and reference elastomers. The dash line indicates the trade-off between toughness and stiffness for the reference materials. FIG. 4d demonstrates the elastic behaviour of the polymers of the invention. Even at different stresses and strains, the polymer is elastic and returns to its original form when the deformation forces are removed.

[0123] FIG. 5. (a) shows a typical cyclic loading/unloading curves of PDUE. The top line is the loading curve, while the lower line is the unloading curve. The shaded area between the loading and unloading curves denotes the energy dissipation. Recovered strain is denoted by the black arrow. (b) Energy dissipations (left hand columns) and dissipation ratios (right hand columns) for PDUE examples at different maximum strains. The dissipation ratio (right hand columns) is the ratio between energy dissipation and loading energy (i.e., the area under the loading curve). (c) Recovered strains (left hand column) and shape recovery ratios (right hand columns) at different maximum strains. The shape recovery ratio is the ratio between recovered strain and maximum strain.

[0124] FIG. 6. shows the melting point of the PDUE. The peaks on the reversing curve and non-reversing curve are attributed to the melting point of the hydrogen bonding domain (roughly)77? C.).

DETERMINATION METHODS

Determination of Fracture Toughness and Young's Modulus

[0125] The fracture toughness (T) is characterized by the pure shear test proposed by Rivlin and Thomas (Rivlin, R. S. & Thomas, A. G. Rupture of rubber. I. Characteristic energy for tearing. Journal of Polymer Science 10, 291-318, doi: 10.1002/pol.1953.120100303 (1953)), which has been widely adopted to characterize fracture of soft materials. The experiments were conducted on a uniaxial tensile test machine (Instron 5944 with a 2 kN load cell). A notch of 15 mm in length was made in a rectangular specimen of the material (Width: ?60 mm; thickness: ?1 mm), as shown in FIG. 1a. Specimens with and without a notch were fixed in two clamps with a distance of 10 mm between them, and then subjected to uniaxial tensile testing with a loading rate of 0.005 mm/mm/s. The fracture toughness (?) is given by:

?=W.sub.CH (S1)

where W.sub.c is the integrated area under the stress-strain curve of the unnotched sample in the region from zero strain up until the critical strain (which is the failure strain of the notched sample). His the distance between the two clamps.

[0126] The Young's modulus (E) of the elastomers was calculated from the gradient of a straight line fitted to the stress-strain curve of an unnotched sample of the elastomer in the initial region (strain from 0 to 0.05. i.e. in the region where the strain is approximately zerosee FIG. 1b).

In Situ Scanning Electron Microscopy (SEM) During Stretching

[0127] A sample of the polymer film was cut using a homemade stamp-type cutter to obtain a dog-bone specimen (gauge section: 3 mm?4 mm). Afterwards, a sharp pre-crack (length: 0.95 mm) was introduced to the gauge section using a craft knife. Tensile testing was conducted on the specimen in a field emission scanning electron microscopy (FE-SEM). The displacement was applied to the specimen while observing around the crack.

Transmittance

[0128] Transmittance of samples was measured by a spectrophotometer (Cary 14 UV/Vis/NIR) in the wavelength range from 400 nm to 800 nm.

Thermogravimetric Analysis (TGA)

[0129] Thermogravimetric analysis was conducted by using a Netzsch instrument (TG 209F1 Libra). The sample was placed in the crucible and heated from 30? C. to 800? C. with a heating rate of 10? C./min under nitrogen atmosphere.

Modulated Differential Scanning Calorimetry (MDSC)

[0130] DSC experiments were carried out by using a Netzsch instrument (DSC 214 Polyma) at a heating ramps of 5? C./min, and a modulation amplitude of 0.5? C. with a period of 60 s.

Experimental Part

Synthesis of Polymer

[0131] Several polymers according to the present invention were prepared and subjected to mechanical testing. The prepared polymers are termed PDUE5000, PDUE3000 and PDUE1000 respectively. Scheme 1 shows the reactions effected:

##STR00010##

[0132] PDUE5000 was synthesised as follows. Aminopropyl terminated polydimethylsiloxane (NH.sub.2-PDMS-NH.sub.2, 75.000 g, 15 mmol, molecular weight: 5000 Dalton, Gelest) was dissolved in tetrahydrofuran (THF, 150 mL, Sigma-Aldrich). The solution was then added dropwise into isophorone diisocyanate (IDI, 6.6684 g, 30 mmol, Sigma-Aldrich) in THF (45 mL) under vigorous stirring. After stirring for 2 h, 0.5 mL of the mixture was pipetted out and evaporated for characterization (the intermediate is termed PDUE-m herein), then 1,2-bis(2-aminoethoxy)ethane (2.223 g, 15 mmol, chain extender, Sigma-Aldrich) in THF (30 mL) was added into the remaining mixture under vigorous stirring. Afterwards, another 20 mL of THF was added into the mixture to lower the viscosity and kept.

[0133] After stirring for another 48 h, the viscous mixture was stored for further fabrication or testing.

[0134] PDUE3000 and PDUE1000 were prepared followed the same procedureonly using an aminopropyl-terminated PDMS having a molecular weight of 3000 and 1000 Dalton respectively.

[0135] The molecular structures of PDUE-m and PDUEX (where X is the molecular weight of the aminopropyl-terminated PDMS) were confirmed by means of Fourier transform infrared (FTIR) and .sup.1H nuclear magnetic resonance (NMR) spectroscopy. Thermogravimetric analysis showed no clear degradation below 250? C. (see FIG. 3). The films were clear upon visual inspection and had a transmittance of greater than 90% over the range 400-800 nm when measured according to the transmittance method described herein.

Comparative Polymer (CE1-PDUQ)

[0136] A comparative polymer was prepared by following the synthesis described above using diaminopropyl terminated polydimethylsiloxane (NH.sub.2-PDMS-NH.sub.2, 75.000 g, 15 mmol, molecular weight: 3000 Dalton, Gelest) as the PDMS macromonomer and isophorone diisocyanate (IDI, 6.6684 g, 30 mmol, Sigma-Aldrich) as the diisocyanate but without using a chain extender. The fracture toughness and stiffness of this polymer are reported in Table 1. The stiffness and toughness of this polymer is much lower than that of the inventive polymers.

##STR00011##

Fabrication of PDUE Films for Testing

[0137] 25 mL of the polymer mixture obtained by the method outlined above was poured into a homemade Teflon pool mould with a dimension of 7?7?1 cm.sup.3, covered by petri dishes and allowed to dry under ambient conditions for 5 days. At the end of this period, the elastomeric film formed was gently peeled from the mould. Tensile testing was then performed on samples of the polymer film (and on samples of comparative polymer compositions) according to the method described under Determination methods. The results of this testing are shown in the table below.

TABLE-US-00001 TABLE 1 Fracture toughness Young's modulus Sample ID (J/m.sup.2) (MPa) PDUE5000 16434 3.2 PDUE3000 17016 10.3 PDUE1000* N.A. N.A. (comparative) CE1 6854 1.6 *during the synthesis of PDUE1000, precipitation occurred when adding the chain extender, thereby leading to failure of the synthesis.

[0138] As shown by Table 1, the polymers of the inventive examples have unexpectedly high fracture toughness and stiffness (as indicated by the high Young's modulus) when having a structure according to the present invention. In general, there is an inverse relationship between fracture toughness and stiffness, and so there is often a trade-off between these two properties. On the contrary, the inventive examples have high fracture toughness and stiffness. The fracture toughness of PDUE3000 and PDUE5000 is much higher than that of other polymers having equivalent stiffness.

[0139] Furthermore, PDUE3000 surprisingly has a Young's modulus more than 3 times higher than that of PDUE5000, without exhibiting a corresponding drop in fracture toughness (the fracture toughness is in fact slightly improved). This unexpected finding forms an additional aspect of the present invention.

[0140] The comparative polymer example PDUE1000 on the other hand, having a value of n according to formula (A) of approximately 11 (i.e. less than defined by claim 1), was not successfully synthesised. Without wishing to be bound by theory, it is believed that the failure of the synthesis of PDUE1000 is due to the high concentration of hydrogen bonding sites when using such a short PDMS segment, thereby leading to aggregation and subsequent precipitation of polymer chains in the solution. The high fracture toughness and stiffness of the inventive examples on the other hand is believed to be due to the formation of strong hydrogen bonding domains, which may act to dissipate energy when the material is stretched.