How it depends on the size of the molecular size like as where in smectic phase layers distance equal to the molecular size.
R. Tota: I am answering your question by using 2D XRD pattern of nematic phase. The small angle 2D XRD pattern of a typical nematic phase consists of two diffuse spots (or crescents or lobes) in the small angle regime as well as two diffuse spots in wide angle regime. The wide-angle diffuse spots will be oriented normal to to small angle diffuse spots. The d-spacing calculated from small angle diffraction peak approximately corresponds to the length of molecule. Whereas, d-spacing (usually 4.5 Angstrom) calculated from wide angle diffraction peak provides the inter-molecular separation (i.e. hydrocarbon chain diameter). In other words, the d-spacing corresponding to wide angle diffraction peak gives the approximate width of molecule. So, if you increase/decrease the size of your molecule then you will get the increased/decreased d-spacing corresponding to small angle XRD in the nematic phase.
In the smectic phase, due to layered structure, the small angle XRD shows diffraction peaks alongwith higher order harmonics. The d-spacing corresponding to first small angle diffraction peak corresponds to layer periodicity (i.e. layer spacing). If the molecules (better to say director n) of smectic phase (for ex.SmA) are parallel to layer normal (k) then layer spacing measured from small angle peak would correspond to size of the molecule. If the layer spacing is less than molecular size then smectic phase would be a tilted phase (for ex. SmC). Moreover, one can easily calculate the tilt angle of SmC phase by knowing d-spacing values of SmA and SmC phases.
Thank you Dr. Gautham singh for your valuable answer. Really i got some clarity to under stand differences in XRD pattern of the smectic and nematic phases. The main difference between their ordering is observed by higher order hormonicity in smectic got by your answer. If you dont mine can i ask some questions regarding this.
1) why 2D XRD could give information? what is the problem with normal small angle XRD (smectic phases normal XRD enough)?
2) Why we are not getting information about the width of the molecule of for smectic phase molecules? is it possible to measure by 2D XRD for smectic phase molecule?
Actually my basic back ground is chemistry, i hope i will get response from you.
And also you can suggest some papers , reviews or books regarding this.
Thanking you.
Hi,
The nematic phase of small angle XRD shows a slightly diffused peak, unlike some sharp peak for ordered smectic phases. More details about nemtaic phases, including their cybotactic cluster containing smetic arrangements can be founf from the papers Adv. Funct. Mater., 2012, 22, 1671-1683, ChemPhysChem., 2014, 15, 1323-1335. Hope mentioned papers can make you understand better. Best wishes
Shanker G
R. Tota: The d spacing corresponding to wide angle X-ray diffraction peak will give you the width of molecule in smectic phase too. Its just a matter of the detector that you choose to record the X-ray diffraction in SAXS or SAXS/WAXS mode. To acquire 2D XRD patterns, one has to use Area detector and point detector for 1D XRD. You can get information about layer spacing from 1D XRD measurements too. Actually, in 2D XRD, you have to first extract the Scattering intensity (I) versus scattering vector (q) from 2D pattern and then you will evaluate the d spacing corresponding to diffraction peak. However, you will directly get I versus 2 theta or q in 1D XRD measurements and then evaluate d spacing. For reference, you might consult the Chapter 3 of the book Liquid Crystals: Experimental Study of Physical Properties and Phase Transitions edited by Prof. Satyendra Kumar, Kent State University, USA. You can also find many review articles and book chapters (Handbook of liquid crystals) on X-ray diffraction studies on liquid crystals in the literature.
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