Application of laser radiation for investigation of

Application of laser radiation for investigation of oriented polypropylene membranes Alexander A. Zinchika, Ivan S. Kuryndinb, Ksenia V. Ezhovaa, Galina K. Elyashevichb National Research University of Information Technologies, Mechanics and Optics (ITMO University), Department of Physics, Saint-Petersburg, Russia b Institute of Macromolecular Compounds, Russian Academy of Sciences, St.Petersburg, Russia a

ABSTRACT The oriented microporous polypropylene film membranes have been prepared in the process based on the melt extrusion. Functional characteristics of the membranes (permeability, overall porosity, sizes of pores) were controlled by the parameters of the preparation process. The samples had a well-developed porous structure and contained a through flow channels. The structure of the films was investigated by laser light scattering in dependence on the orientation degree. Light scattering patterns have been obtained using a low-energy He-Ne laser with the power of 5 mW and the wavelength of 633 nm. The optical setup also included the beam-forming system, and the detection unit connected to a PC. To detect the light scattering pattern, a lens less CCD-camera was used. The scattering patterns have a developed speckle structure, and, therefore, to simplify further studies, intensity should be averaged over a sufficiently large number of patterns using a special computer program. These scattering patterns are characterized by a specific type of symmetry and differ from any patterns typical for oriented crystallizable polymers. It is found that similar patterns are observed for all porous samples regardless of their orientation degree. The size of central maximum of the scattering pattern is dependent on the polymer film orientation degree. The results correlate well with the dependence of the porous films overall porosity on orientation degree. Keywords: polypropylene, microporous membranes, light scattering, permeability.

1.

INTRODUCTION

Numerous porous systems containing microscopic pores have been prepared and studied by now. Polyolefin films polyethylene (PE) and polypropylene (PP) are the very promising commercial polymers due to their wide-spreading and easy manufacturing1-4. Owing to high chemical resistance to various media, they are widely used as membrane materials in medical, chemical, and food industries. Microporous polyolefin films have a number of advantages over porous systems based on inorganic substances, namely: small thickness and hence lower resistance to mass transfer, efficiency and simplicity of the production process, and high elasticity. Polyolefin microporous films are the most attractive membrane material because they contain well-developed network of through flow channels which provides permeability for liquids and gases. The value of the permeability depends on the sizes and number of these channels, i.e., the volume structural characteristics of the material which are controlled by the parameters of the porous structure formation process. Information on the structure of porous films can be obtained by different nondestructive structural methods such as X-ray scattering and sound propagation5. Optical methods (birefringence, dichroism) were used for investigation of photosensitive and phorochromic composites based on PE porous films6. In this work the optical scattering was chosen to investigate the influence of orientation parameter in the preparation process of the films on their structure. The subject of our research was polypropylene (PP) microporous films. In our works7 the optical investigation was used to analyze the porous structure of PE porous films. The aim of this study was to find a correlation between the functional characteristics of porous structure (porosity, permeability) and parameters of an optical band light scattering. The analysis of optical scattering from porous samples is of special interest because control devices can be installed directly into the technological line for preparation of membranes.

Optical Micro- and Nanometrology VI, edited by Christophe Gorecki, Anand Krishna Asundi, Wolfgang Osten, Proc. of SPIE Vol. 9890, 989016 · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2227776

Proc. of SPIE Vol. 9890 989016-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/04/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

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OBJECTS AND METHODS OF INVESTIGATION

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Porous PP films under study were prepared in the multi-stage process based on the melt extrusion. The process includes the following stages8 -11: melt extrusion, isometric annealing, uniaxial extension at room temperature (pore formation stage) and thermal fixation. At the last stage the resulting porous structure is fixed by keeping under isometric conditions at an elevated temperature for the relaxation of internal stresses accumulated in the process of extension. The film porous structure formation is controlled by a number of thermal and orientation parameters. Annealing of the samples crystallized at high velocities of the melt flow at the crystallization temperatures well below the melting temperature results in the formation of “hard elastic” type systems. Along with the high modulus of elasticity lying between the module of isotropic and highly oriented samples, these systems exhibit also a considerable elastic recovery typical for elastomers.

Figure 1. Structure model of PP samples at different stages of the porous structure formation: a) extruded sample; b) annealed (hard elastic) sample; c) porous structure formation at uniaxial extension (the extension direction is shown by the arrow); L01 is the long period in the extruded sample, L02 is the long period in the annealed sample, b1 - the lamellar thickness in the initial sample and b2 - the lamellar thickness in the annealed sample.

These properties of hard elastics result from their supermolecular structure which consists of rather large folded-chain crystals (lamellae) located parallel to each other and perpendicularly to the melt flow direction and along the preferential orientation of polymer chains in the sample (Fig. la, b). Lamellae are connected by a relatively small number of tie chains (“bridges”) that are statistically distributed over the lamella surfaces. Extension of hard elastic samples in the orientation direction (at the pore formation stage) leads to bending and moving apart of lamellae between the tie chain bridges (Fig. 1c), thus giving rise to discontinuities (pores). At the large extensions, the discontinuities coalesce, thereby forming through channels, and the porous sample becomes permeable to liquids and gases (Fig. 1c)9-11. At the first stage, extrusion, crystallization at a high velocity of melt flow takes place, the sample orientation depending on the melt orientation degree - spin draw ratio λ. The second stage, i.e., annealing at the temperature close to the polymer melting temperature under the conditions preventing shrinkage of the sample, leads to an additional increase in the orientation degree of the samples which acquire hard elastic properties (Fig. lb). Both the extruded and, especially, annealed samples, being oriented systems, are highly transparent. At annealing in isometric conditions the orientation degree of the samples increases noticeably. Porous structure is formed at the third stage of the process, in uniaxial extension of hard elastic samples. In the result of the pore formation at this stage the samples become opaque (white milk) due to the light scattering on the walls of pores. Commercial grades of isotactic PP (Mw = 380000, Mw/Mn = 4-5, Tm = 172 0C) were used to obtain the porous films. The films formation at the stage of polymer melt extrusion was performed using a flat die. The melt was crystallized in air. The degree of melt orientation was controlled by the spin draw ratio λ. The samples with λ ranging from 30 to 85 have been prepared. Extruded films were annealed at 1700 C. Uniaxial extension of annealed films was performed at room temperature. The degree of extension at this stage was 200%. The thickness of the porous films decreased from 43 to 17 μm at increasing of λ in the mentioned range.


The overall porosity of the samples was found by the gravimetric method and was calculated as P = [(ρ-ρp) /ρ] ⋅ 100 %, where ρ is the density of dense (nonporous) PP films, which is 900 kg/m3. Permeability of porous films was determined by flow velocity of the wetting PP liquid (ethanol) in the filtration cell. Laser light scattering method was used for the investigation of PP microporous membranes structure. The patterns were obtained with a low-energy He-Ne laser with the power of 5 mW and the wavelength of 633 nm and a lens less CCDcamera. The optical setup also included the beam-forming system and the detection unit connected to a PC. To simplify studies, intensity should be averaged over a sufficiently large number of patterns using a special computer program.

3.

EXPERIMENT

3.1 Characterization of polypropylene porous membranes The resulting porous film has a complicated multi-scale structure. Fig. 2 shows a SEM image of PP membrane which demonstrates the oriented character of the structure and periodicity in the alteration of structure details of the dense polymer and pores between them.

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Figure 2. SEM images of surface of PP membrane.

The porous films under study have the pores of three types, i.e., surface, inner, and through ones. All these pores contribute to the overall porosity. But permeability is provided only by the through pores. Fig. 3 shows the dependences of overall porosity (fraction of the volume occupied by pores) and permeability on orientation (melt spin draw ratio)11, 12. It is seen at Fig. 3 that there is the threshold magnitude of λth ∼ 20 when permeability appears in the sample, and it evidences that the through channels connecting the film surfaces are formed in the sample. The value of porosity reaches ∼ 23 % at λ = λth.. The threshold value for controlling parameter indicates that through pores are formed in these porous systems by the percolation mechanism. At threshold magnitude of λth the porous structure character changes. At spin draw ratios lower than λth the pores are in the form of isolated cavities, the sample is impermeable. At λ > λth the cavities coalesce, the number and sized of through pores increases and permeability grows (curve 2 in Fig. 3). It is seen that the pore coalescence process does not affect the overall porosity (curve 1 in Fig. 3).


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To determine what changes in the porous structure of PP films are responsible for their permeability, the number and sizes of through channels were measured by filtration porosimetry and were calculated by the Poiseuille method13. It is seen in Fig. 3 and Table 1 that permeability grows with increasing λ due to two factors, i.e., formation of new through channels and increase in their sizes. Table 1. Effect of spin draw ratio on the number and sizes of through channels.

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3.2 Laser light scattering investigations For the experimental study of the scattering of laser radiation in the PP porous membranes used installation, which is shown in Fig. 4.

Figure 4. Scheme of the experimental set up.

An installation used for investigations of the film surface is represented on Fig. 4. The installation includes a source of radiation (He-Ne laser with wavelength 632,8 nm and power 5mWt) with the beam-forming system (1), light filters (2) and a polariser (3) for precise control of the intensity of a laser beam. The round aperture (4) transmits only main laser beam. The lens (5) makes it possible to obtain a sharp image in plane of the CCD-matrix when the object of research is


absent. Sampple holder (6) with w the sampple is placed to t a minimum m distance of thhe CCD, whicch allows the registration of the scatteringg at large angles. A circuit to record of the scattering g at large angles is needed because the dimensions of inhomogeneitties in polym mer membrannes have a scale of miccrometers, annd thus the scattering an ngles will bee approximatelyy twenty degrees. The fram me permits too rotate the saample aroundd the optical aaxis of the ap pparatus in thee range 0÷3600 with a minim mum step of 10, which is sufficient s for the studies. Moreover, M thee frame allow ws to move thee sample in the plane perpeendicular to the t optical axxis of the two coordinatess up-down, riight-left with a positioningg accuracy of 0.1 0 mm (sampple has dimenssions of 20x20 mm). The sample s is placed between tw wo flat optical glasses. Thiss frame design gives the oppportunity to exxplore the dry sample and th he sample plaaced in the im mmersion liquid d, both. Videoo camera with CCD C and autoomatic sensitivvity adjustmennt (7) was useed for registrattion of the scaattering patterrn. To increasee the angular fiield of view, thhe lens has beeen removed from f the cameera. The videocam mera gives a possibility too visually obsserve the scaattering patternn in the dynaamic mode. The T scatteringg patterns havee a developed speckle structture, and, therrefore, to simp plify further studies, intensiity should be averaged overr a sufficiently large numberr of patterns using a special computer pro ogram. When the lasser source is used the scaattering patterrns obtained from f polymerr membranes have a deveeloped specklee pattern, and a section of thhe scattering pattern p has a sharp s intensity y jumps that make m it difficuult to study it.. The standardd methods for removing r speckle image byy smoothing in i the image editor e have noot given the ddesired result.. Therefore, too make further investigationss more reliable the intensityy was averaged over a sufficciently large nnumber of imaages. This method of adding scatttering films shot s in the neaar sample poin nts has the folllowing advanttages: a) It is the eaasiest way to increase the signal s to noise ratio as random speckless, and the chaaracteristic scaattering angless remain constaant. b) The size of o the beam is i small, so thhe impositionn of scattering g patterns, takken from diffe ferent points of o the samplee, provides the information i avveraged over the t sample. The amount required r for thhe averaging of o images is determined d ex xperimentally. The optimum m value is in the t range 8-100 images. Beforre fixing eachh of the next im mages the object is moved a few millimeeters in up-dow wn or left-righ ht direction, at a a constant diistance from the t camera. Fig.5 F presentss the averaged scattering patterns p for thhe porous film m obtained at a λ=30 (a), 44(bb), 63(c), and 85(d). A1,1. `'s

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Figure 5. Averaged A imagges of light scatttering for the poorous PP samplles obtained at λ=30 λ (a), 44(b)), 63(c), and 85((d). The orientationn direction is veertical.


Figure 6. Central C cross-seections of averaaged image of light l scattering: horizontal crosss-section (a) annd vertical crosss-section (b) (λ=44).

All the patterrns demonstraate a strong annisotropy of thhe samples. It can be suppoosed that the hhorizontal crosss-section of a small angularr width is relaated to the sccattering from m “bridges” (th he ties conneccting lamellaee) which are aligned in thee orientation diirection and reemain unaltereed during exteension (Fig. 1c). At the sam me time the m maxima of the vertical crosssection has a large angularr width are likkely related to scattering fro om lamellae. Fig. F 6 shows ccentral cross-ssections of thee averaged scatttering patternn (Fig. 5) in thhe horizontal and a vertical planes.

Figure 7. Angular A width of the central maxima m θ of thee averaged apprroximated scatteering pattern ass function of λ.

Using the crooss-section off averaged scaattering patterrn the angularr width of thee central maxiima θ were deetermined andd plotted. Fig. 7 shows the deependence of θ on the orienntation degreee λ. The graphh shows the rappid growth off θ in the rangee λ from 30 to 60, 6 and a slighht increase at its higher valuues. It is seen at a comparison of o the changess of θ with λ and a the analog gous curve for overall porossity (Fig. 3) th hat both graphss w values of λ and a the more weak w dependeence in the reggion of large values v of λ. show a rapid growth in the region of low

4. CONCLUSI C ION l scatterinng. The observ ved scatteringg Polypropylenne microporouus membraness were investtigated by thee method of light patterns are characterized c b a specific type of symm by metry and diffe fer from any patterns p typicaal for oriented d crystallizablee polymers. It is i found that similar patterrns are observved for all porrous samples regardless off their orientattion degree. It I was shown thhat the size of o central maaximum of the scattering pattern p is deppendent on thhe polymer fillm orientationn degree. For the first time t a comparrison of the chharacteristics of o light scatteering pictures and the valuees of overall porosity for thee polymer poroous films wass carried out. The results of o this study allow a establishing the correlation betweeen the opticaal scattering parrameters and the t volume chharacteristics of the orienteed polymer fillms. It is provved that the deependences onn orientation degree d for thee optical picttures data andd overall porrosity measurred by other (non-optical) experimentaal


techniques are in a good agreement with each other. It is the evidence that the optical patterns are formed by the light scattering on the interface pores and dense polymer material. The correlation between the optical and functional characteristics allows the use of optical methods and image processing techniques to instantly control in technological processes of the polymer film membranes manufacturing. Moreover, there is a possibility to automatize the nondestructive testing of surface.

ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (Project №16-03-00265a).

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