WO2008109968A2

WO2008109968A2 - Non destructive monitoring of polymer dynamics

Info

Publication number: WO2008109968A2
Application number: PCT/BE2008/000016
Authority: WO
Inventors: Jean-François Feller; Mickaël Castro; Frédéric LUIZI
Original assignee: Nanocyl S.A.; Universite De Bretagne Sud
Priority date: 2007-03-13
Filing date: 2008-03-13
Publication date: 2008-09-18
Also published as: WO2008109968A3

Abstract

The present invention is related to the non destructive monitoring of polymer dynamics through the measurement of electrical conductivity variation, such electrical conductivity being provided by the addition of carbon nanotubes in the polymer. Indeed, a change of the glass transition temperature will results in a change of modulus, of polymeric chain mobility and of free volume. Such change will affect the structure of the percolated network of carbon nanotubes resulting in a change of the polymer resistivity. Hence, the electrical conductivity can be correlated to the dynamic behavior of a polymer within the rubber elastic region.

Description

Non destructive monitoring of polymer dynamics

Field of the invention [0001] The present invention is related to the field of material science and more precisely to the monitoring of the dynamic behaviour of polymers .

State of the art [0002] The thermodynamic principles on which the invention is based are the following. For any polymeric matrix, the rubber elastic region is characterized by a variable modulus of a given polymer. The change of modulus corresponds to a change of mobility of the polymeric chains. The change of mobility of the polymeric chains results in a variation of the free volume within the polymer .

[0003] Besides, carbon nanotubes (CNT) exfoliated into a polymer form a network which, at a given density or weight percentage, will result in electrical conductivity. The relation between the concentration of carbon nanotubes and the resistivity of a polymer is the percolation curve. Such percolation curves can be characterized by three regions. The non conductive region, is characterized by low content of carbon nanotubes forming a network which is not sufficiently dense to ensure percolation and conductivity. The antistatic and conductive region is characterized by a content of carbon nanotubes which is sufficient to form a percolation network resulting in fast increasing conductivity. The third region is characterized by a fully percolated network of carbon nanotubes. Therefore additional loading is having a limited effect on the conductivity. The second and third regions are separated by what is referred hereafter as the percolation threshold. [0004] Hence, the conductivity of a polymer loaded with carbon nanotubes will only change if the loading is increased within the two conductive regions. Correlating the loading of CNT with the resistivity will be best established in a range of 50% loading under or above the percolation threshold as defined in the present invention. Within this range the conductivity will vary from antistatic to highly conductive but with a direct correlation between the CNT loading and the resistivity.

Aims of the invention

[0005] The present invention aims to develop a simple method to assess the dynamic behavior of a nanocomposite. Dynamic behavior of a nanocomposite is depending on the macromolecules dynamics of which it is made .

[0006] The dynamic status of a polymer can be characterized according to the temperature, defining three regions, the glassy region, below the glass transition temperature, the rubber elastic region, between the glass transition temperature and the fusion temperature and the flow region, above the melting temperature. From a dynamic point of view the key region is the rubber state. However, characterizing the faith of the polymer within this region requires a lot of crossed measurements and can rely on many variables ranging from dynamic mechanical characterization, rheological to thermal characterization. Furthermore most characterization methods are destructive or do not allow to monitor tjfee nanocomposite behavior in different environments . [0007] In this context, the objective of this invention was to develop an easy, non destructive method for the in-situ monitoring of the dynamic behavior of nanocomposite using carbon nanotubes as conductive nano filler.

Summary of the invention

[0008] In the frame of this invention, we have demonstrated that another way to change the resistivity beside adding or retrieving carbon nanotubes is a modification of the polymer within the rubber elastic region. Indeed, as shown above, a change of the glass transition temperature will results in a change of modulus, of polymeric chain mobility and of free volume. Such change will affect the structure of the percolated network of carbon nanotubes resulting in a change of the polymer resistivity. Hence, the electrical conductivity can be correlated to the dynamic behavior of a polymer within the rubber elastic region. [0009] The dynamic behavior of a polymer can be affected not only by temperature but also by any modifications of various environmental parameters to which the polymer is sensitive. For instance, the presence of solvents will affect the free volume and thereby the modulus of a polymer. Such changes will results in a modification of the rubber elastic region. So, as demonstrated in this invention, it then becomes possible to monitor such environmental modifications based on their effect on the rubber elasticity of the polymer which will influence the percolation network of carbon nanotubes and therefore be monitored through changes of electrical conductivity.

[0010] ^ As such carbon nanotubes become "dormant" sensors or "in situ probes" of the dynamic behavior of the polymer. Indeed, the modification of the organization of the carbon nanotubes percolated network within a given range of conductivity corresponding to the 50% above or under the percolation threshold as defined herein, as triggered by modification of the dynamic behavior of the polymer in rubber elastic region induced by environmental changes will be monitored by measurements of the electrical resistivity. In the detailed description below, we have shown that such method is non destructive, reversible, reproducible and applicable to a very wide range of polymeric matrix.

Short description of the drawings

[0011] Fig. 1 represents a schematic standard percolation curve demonstrating the relation between carbon nanotubes content in a polymer and electrical resistivity

[0012] Fig. 2 represents percolation curves of various polymers used to support the present invention

[0013] Fig. 3 represents the evolution of the specific volume of the 8 selected polymers as a function of temperature (⁰C)

[0014] Fig. 4 represents the responses to temperature of- POE-3%CNT and PCL-3%CNT' conductive polymer compound [0015] Fig. 5 represents the responses to temperature of different POE-3%CNT conductive polymer compound

[0016] Fig. 6 represents the description of the device developed to modify the dynamic behaviour of polymers using vapour exposure

[0017] Fig. 7 represents the change of resistivity as a result of CNT loaded polymer exposed to different solvents valour reducing the Tg in a different manner. [0018] Fig. 8 represents the change of resistivity as a result exposure of a polymer to different vapour concentrations

[0019] Fig. 9 represents the change of resistivity and sensitivity to dynamic behaviour of a polymer as induced by solvent vapour exposure at different loading level of CNT

[0020] Fig. 10 represents the change of resistivity as a result of modification of the position on the percolation curve using a secondary conductive additive

Detailed description of the invention

[0021] As stated above, the objective of this invention was to develop an easy, non destructive method for the in-situ monitoring of the dynamic behavior of nanocomposite based on the measure of the nanocomposite electrical conductivity where the conductive fillers are carbon nanotubes . [0022] We have demonstrated that another way to change the resistivity beside adding or retrieving carbon nanotubes is a modification of the polymer within the rubber elastic region. Indeed, as shown above, a change of the glass transition temperature will results in a change of modulus, of polymeric chain mobility and of free volume. Such change will affect the structure of the percolated network of carbon nanotubes resulting in a change of the polymer resistivity. Hence, the electrical conductivity can be correlated to the dynamic behavior of a polymer within the rubber elastic region. [0023] The dynamic behavior of a polymer can be affected not only by temperature but also by any modifications of various environmental parameters to which the polymer is sensitive. For instance, the presence of solvents will affect the free volume and thereby the modulus of a polymer. Such changes will results in a modification of the rubber elastic region. So, as demonstrated in this invention, it then becomes possible to monitor such environmental modifications based on their effect on the rubber elasticity of the polymer which will influence the percolation network of carbon nanotubes and therefore be monitored through changes of electrical conductivity.

[0024] As such carbon nanotubes become "dormant" sensors or "in situ probes" of the dynamic behavior of the polymer. Indeed, the modification of the organization of the carbon nanotubes percolated network within a given range of conductivity corresponding to the 50% above or under the percolation threshold as defined herein, as triggered by modification of the dynamic behavior of the polymer in rubber elastic region induced by environmental changes will be monitored by measurements of the electrical resistivity. In the detailed description below, we have shown that such method is non destructive, reversible, reproducible and applicable to a very wide range of polymeric matrix.

Conductive network of CNT in the polymer

[0025] State of the art information available notably in a multitude of publications have demonstrated the relation between carbon nanotubes content in a polymer and the electrical conductivity or resistivity. Such relation is expressed as the percolation curve as shown in figure 1. In the present invention, percolation curves have been established in standard polymer films of polyamide 12

(PA 12) , polycarbonate (PC) , polyethylene terephthalate

(PET) , polylactide (PLA) , polycaprolactone (PCL) , polyethylene oxide (PEO) , polymethylmethacrylate (PMMA) , polystyrene (PS) . The curve and the resulting percolation point as defined above have been plotted as presented in figure 2.

Dynamic behavior of the polymer [0026] As presented above, the dynamic behavior of a polymer affects its rubber elastic temperature range resulting in changes in viscoelasticity, polymeric chain mobility and free volume. Hence, the dynamic behavior of a polymer can be expressed as a function of its specific volume. In figure 3, the evolution of the specific volume of the various polymers testes as a function of the temperature is presented. As shown, two polymers were selected for their low melting temperature (PEO and PCL) and the other polymers selected have similar characteristics. The melting temperature appears clearly on the figure where there is a sharp increase in specific volume. As for the glass transition temperature (Tg), it is presented in the Table 1. Again the polymers have been selected to cover a wide range of Tg thereby showing the versatility of the invented monitoring method.

Table 1: Glass transition temperature (Tg) for the 8 polymers selected

Limit of the applicability of the method

[0027] The method of monitoring the dynamic behavior of the polymer developed in the present invention is limited to the rubber elastic region as shown in figure 4. Indeed, in PEO and PCL which have low melting temperature as shown in figure 3, there is a change of conductivity that can be directly linked to increased temperature until melting temperature . When the temperature increases beyond the fusion temperature, the carbon nanotubes network become totally disorganized, resulting into a shrinkage of the network in the fluidized polymer thereby locally increasing the density of the conductive filler and increasing conductivity. Such drastic change of the conductive network is irreversible if only one polymer is used while the present invention claims a non destructive monitoring method. Hence, the melting temperature is the upper limitation of the applicability of the method. Therefore, to overcome this limitation, it is possible to use two polymers systems having co-continuous structure and prepared in such a way that the polymer in which the nanofillers are dispersed can melt whereas the second one of higher melting temperature is still in the solid state. In fact it must be noted that it is a unique method to evaluate macromolecules motion due to Brownian movement in the liquid state. In such conditions and even if the CNT network can reyersibly be disconnected and reconnected upon heating/cooling cycling, preferred conditions to monitor conformational changes of macromolecules are when the melting temperature of the loaded polymer is not reached.

[0028] As for the lower limit of the method, when the polymers are in the glassy state ,i.e., below their glass transition temperatures, no important dynamic behavior is expected. Their conductivity is thus only driven by their percolation curves presented in figure 3, and not by any conformational change of macromolecules. Influence of some environmental parameters on the dynamic behavior of polymer.

[0029] Beside temperature, any other environmental parameters such as type of solvents, mechanical strain, degradation due to breakage, alteration due to exposure to UV, thermal or chemical ageing that affect the viscosity of a polymer or its specific or free volume or its polymeric chain mobility will result in a change of the rubbery state region. For e^'xample, the presence of molecules like that of solvents or plasticizers and their volume fraction with regards to a polymer will largely affect the polymer glass transition temperature, decreasing it to tend towards the glass transition temperature of the solvent as presented in table 2.

0 ε 146, 115702 114, 269819 192 ,51714 128 ,411294 158 ,85947 117, 428297

0 0 140, 876494 108, 176432 190, 742358 122 ,549769 154, 302671 111, 366601 i 136 102,7 189 117,2 150 105,9

Table 2 : Evolution of the glass transition temperature (Tg) of the 8 polymers selected as a function of their volume fraction in presence of one of the 6 reference solvents selected

Measure of impact of changes of the dynamic behavior of polymers as a result of electrical conductivity [0030] As shown in figure 4, one way of affecting the dynamic behavior of a polymer is to raise temperature. Other ways include exposure to varying environment. To simulate such environmental variation and thereby reduction of the Tg to a level below room temperature and thereby enter the ^ rubber elastic region at which our non destructive monitoring method is efficient, a specific device was developed as presented in figure 6. Using this device, we were able to expose polymers of various Tg to various volume fraction of either vapors or liquids thereby triggering controlled impacts on the dynamic behavior of the different polymers.

[0031] Using such device, we notably exposed polymer with different solvents under controlled temperature affecting their Tg in a different manner. For example, PC can be exposed to THF which will, at room temperature, reduce the polymer Tg less than Toluene which will lower the Tg more as shown in Table 1. To demonstrate that the monitoring system is none destructive we performed successive changes showing the reversibility of the method. A lot of information is shown from Figure 7. [0032] Firstly, the relation between the dynamic behavior of the polymer and the conductivity due to the presence of a percolation network of carbon nanotubes is shown through the pulsed changing conductivity corresponding to the pulsed exposure to the solvents vapors. Secondly, the reversibility and none destructive nature of the method is clearly shown. Thirdly, the specific shape of the different changes of conductivity reflects the range of sensitivity of the method. Indeed, in the frame of THF exposure, as the Tg of the polymer will be less affected than during toluene exposure, saturation is quickly observed. Indeed, as THF will lower the Tg less, at room temperature, the polymer will be within the rubber elastic region only a short time. Beyond a given volume fraction of the solvent, the Tg of the polymer will no longer be decreased. Hence, the polymer will remain in glassy state and no longer be affected by the presence of additional solvent vapor. That effect is reflected on figure 7 by the saturation of the resistivity change as shown by the plateau reached by the resistivity curve. Nevertheless, the change is reversible and non destructive.

For the exposure to a solvent inducing a higher decrease of the Tg, toluene in the present example, the range of monitoring before the polymer is in a glassy state at room temperature is not reached. Hence, the change of conductivity is linear throughout the exposure.

[0033] The range of efficiency of the method can also be demonstrated by showing the sensitivity of a given polymer to various volume fractions of vapors of the same solvent. As shown in figure 8, the concentration of the solvent triggers resistivity changes that are of various intensities with a linearly increasing of the relative resistivity (referred to as sensitivity) for all the solvent tested. This shows clearly that within the rubber elastic region of the polymer, the method of assessing its dynamic behavior is fully reliable.

Sensitivity of the method as a results of loading of carbon nanotubes [0034] As claimed, the method of assessment of the dynamic behavior of a polymer is efficient is the carbon nanotubes content is ranging from the concentration at the percolation point as defined in the present invention plus or minus 50% percent. For example, as shown in figure 9, the sensitivity of the method as expressed by the relative change of resistivity is higher when the loading of CNT in the polymer correspond to the percolation threshold as described in the present invention. Beyond the defined limit, changes in the CNT loading do not affect notably the resistivity and thereby, the monitoring of dynamic behavior is no longer possible.

Influence of other fillers on the sensitivity of the method [0035] To validate the efficacy of the method in multicomponent nanocomposites, we assess the impact of the addition of another conductive filler in the formulation. Figure 10 presents the effect of various concentration of conductive carbon black on the sensitivity of the system. As shown from figure 1, an addition of 0,5% of multiwall carbon nanotubes corresponds to the percolation point as described in this patent. Any additional conductivity brought by the addition of conductive carbon black lowers the sensitivity as it acts as if it moved the system along the percolation curve, away from the percolation point.

Claims

1. A non destructive method to monitor the dynamic behavior of a nanocomposite comprising a polymer in a temperature range between the glass transition temperature (T_g) and the melting temperature (T_m) comprising the steps of: selecting a nanocomposite comprising carbon nanotubes, within a range of 50% above and 50% below the loading of the carbon nanotubes at the percolation threshold;

- measuring the level of electrical conductivity of said nanocomposite; correlating said level of electrical conductivity with the dynamic behavior of said nanocomposite;

- monitoring the dynamic behavior of said nanocomposite .

2. Method according to claim 1 wherein said method is suitable for monitoring any physical parameters of said nanocomposite.

3. Method as in any of the previous claims wherein said physical parameters are environmental parameters .

4. Method as in any of the previous claims wherein said environmental parameters include temperature, type of solvents, mechanical strain, degradation due to breakage, alteration due to UV exposure, thermal or chemical ageing affecting the dynamic behavior of said nanocomposite .