Nano-dimension pore structure analysis of poly(ethylene terephthalate) knitted materials: An insight combining SEM images

Document Type : Case Study


Department of Textile Engineering, Isfahan University of Technology Isfahan, Iran


Scanning Electron Microscopy (SEM), an easily acquired and widely applied image acquisition and analysis method, have rarely been used to study the pore structure of poly(ethylene terephthalate) knitted fabrics after ultraviolet irradiation simultaneously ozone gas exposure. In this work, we present an investigation of nano-dimension detection of the pores and fractures using SEM observations. The morphological characteristics of the poly(ethylene terephthalate) fabric surface can be revealed by SEM method. By detecting 9 high resolution SEM images, the pores morphology of different scales were acquired. And the studied poly(ethylene terephthalate) knitted substrate shows different types of pores and holes with multi-resolution at the surface layer after ultraviolet irradiation and ozone gas exposure for 80 minutes. This work demonstrated that the combination of two dimensional (2D) SEM results is effective in detection of surface morphology, and is of significance in revealing the pore structure of materials at nano dimension scale. Sub-micron porosity with pore radii as small as 2.5–10 nm was observed in SEM cross-sections. The formation of nano dimensional pores on the surface of the fabric is because of the physical etching ( due to the ion bombardment in the radiation chamber).


Scanning electron microscopy (SEM) is utilized to examine the pore morphology and topography of the surface of materials [1]. This investigation began in a work to explore the changes in the surface morphology and topography of poly (ethylene terephthalate) knitted materials by means of the sample’s SEM images after ultraviolet irradiation and simultaneous ozone gas exposure  for 80 minutes. The main innovation of this paper is twofold:
1) Combining SEM 2D results to comprehensively study the nano pores morphology of poly (ethylene terephthalate) knitted materials, and
2)    Obtaining high-resolution SEM images. 
Moreover, in this regard, to the best knowledge of the authors, papers on the SEM observation of the pore structures on the poly (ethylene terephthalate) knitted materials could not be found in the literature. 

Sample preparation and experimental procedure
The following  fabrics  are  applied  throughout  this exploration:  Pure  poly (ethylene terephthalate)  fabrics  (100  %) with the weight, thickness and Yarn linear density of   18.45  g/m2, 0.781  mm  and  150/144 dtex/filament, respectively.  These fabrics were kindly supplied by Nature Works LLC Company, USA.  
The poly (ethylene terephthalate)  fabrics  were  scoured  with  2  g/L anionic/non+ionic detergent, 1 g/L Kieralon Jet B conc. (non+ionic  surfactant,  BASF)  and  1  g/L  sodium carbonate  (‘soda ash’) at 60  ºC  for 15 min at a  liquor ratio  of  10:  1, and washed  thoroughly. After  scouring,  the fabrics  were  rinsed  with  cold  water  for  10  min  and dried  in  ambient  conditions  to  remove  mill  dirt  and lubricants.
The  poly (ethylene terephthalate)  fabrics  were  irradiated  in  a Ultraviolet/Ozone gas  irradiation  cabinet  for  80 min  (40  min face-up  and  40  min  face-down) (Ultraviolet/O3 radiation cabinet: 11 mW/cm2  intensity UV lamps without outer  envelope  (6 Lamps, made  in Poland)  is placed  in  a  cubic  box with  the side  length  of 60 cm. Strips of samples are placed around the source at a suitable distance  (~2 cm). Atomic oxygen is generated both when molecular oxygen is subjected to the 184.9 nm radiation and when ozone is irradiated at 253.7 nm. The radiation at 253.7 nm is absorbed by most hydrocarbons and also by ozone).
When  the  irradiation  was  completed,  the  sample  was rinsed  adequately  with  cold  water  (30+40  ºC)  at room temperature  for  10  min  in  a  liquor  to  goods  ratio  of 20:1, and then oven dried at 60 ºC for 30 min.

Acquisition of high-resolution images 
In this study, the acquisition of high resolution images was conducted on a XL30MODEL/PHYLIPS Company/Netherland SEM (Fig. 1). 
In order to avoid problems due to charge build-up, the fabrics were previously sputter-coated with gold palladium for two minutes in a SCDOOS  MODEL/Bal+Tech  Company/Switzerland sputter  coating  unit (Fig. 2).  
 In order to reduce the influence of human factors on the experimental results, the following procedures were considered: 
1)    Reducing the waiting time of the intermediate process; 
2)    Scanning more areas for sufficient data;
3)    Using an optical microscope with the instrument to observe the samples firstly and select appropriate areas.
The basic testing principles of the experiment is to scan some of the high resolution images (magnifications of 30000×) (Fig. 3). 

Morphology analysis
We studied the surface topography of poly (ethylene terephthalate) knitted materials by means of SEM observations. The virgin sample exhibited a markedly smooth surface (Figs. 4).

Pore structure
Different kinds of pores are developed on the surface of poly (ethylene terephthalate) knitted fabric after ultraviolet irradiation and simultaneous ozone gas exposure (Fig. 5-c). Some nano-dimension raptures and holes were also detected on the poly (ethylene terephthalate) substrate (Fig. 5-a and b). The pores are well developed in the fabric matrix, and different fractures can also be observed by cutting the matrix (Fig. 5-d and e).
It can be said that the formation of nano-dimension pores, fractures and raptures is presumably due to the physical etching effect of ultraviolet irradiation and ozone gas exposure [2, 3]. Physical etching process brings a highly directional flux of energetic, reactive ions to the surface of the materials [4-6]. By doing so, a precisely controlled patterning of the substrate takes place as the unmasked sample is etched away by the reactive ions [7, 8]. In physical etching, ion bombardment through directional momentum transfer causes the physical sputtering of atoms [9, 10].
Fig. 6 shows the results of other research about the surface analysis of poly (ethylene terephthalate) knitted materials after different irradiations with SEM instrument.
Some other techniques such as Brunauer Emmett Teller method (BET) have extensive application in materials science to compute the solids surface areas through the physical absorption of gas molecules [14,15]. Finally, the author proposes this alternative strategy for the assessment of the surface poly (ethylene terephthalate) knitted materials after ultraviolet irradiation and ozone gas exposure. This strategy is expected to increase the success of surface morphology characterization.

Observations combining SEM tests were conducted to reveal the pore structure of poly (ethylene terephthalate) knitted material. The following conclusions can be drawn:
(1) Combing SEM observations in a nano-scale is effective to reveal pore structure in 2D dimensions. The SEM provides information about pore structure and surface morphology.
(2) SEM images provide information about poly (ethylene terephthalate) knitted material surfaces in a larger observation size with high resolution degrees, which is of significance in revealing pores structures at different scales. 

This study was supported by Isfahan University of Technology. I am grateful to the reviewers for their valuable advice during their busy schedule.

There are no conflicts to declare.

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